Python & JAVA Solutions for Leetcode (inspired by haoel's leetcode)
Remember solutions are only solutions to given problems. If you want full study checklist for code & whiteboard interview, please turn to jwasham's coding-interview-university.
Python and Java full list. ♥ means you need a subscription.
| # | Title | Solution | Basic idea (One line) |
|---|---|---|---|
| 1 | Two Sum | Python Java | 1. Hash O(n) and O(n) space. 2. Sort and search with two points O(n) and O(1) space. |
| 2 | Add Two Numbers | Python Java | Take care of the carry from lower digit. |
| 3 | Longest Substring Without Repeating Characters | Python Java | 1. Check every possible substring O(n^2) 2. Remember the character index and current check pos, if character index >= current pos, then there is duplicate |
| 4 | Median of Two Sorted Arrays | Python Java | 1. Merge two sorted lists and compute median, O(m + n) and O(m + n) 2. An extension of median of two sorted arrays of equal size problem |
| 5 | Longest Palindromic Substring | Python Java | Background knowledge 1. DP if s[i]==s[j] and P[i+1, j-1] then P[i,j] 2. A palindrome can be expanded from its center 3. Manacher algorithm |
| 7 | Reverse Integer | Python Java | Overflow when the result is greater than 2147483647 or less than -2147483648. |
| 8 | String to Integer (atoi) | Python Java | Overflow, Space, and negative number |
| 9 | Palindrome Number | Python Java | Get the len and check left and right with 10^len, 10 |
| 12 | Integer to Roman | Python | Background knowledge Just like 10-digit number, divide and minus |
| 13 | Roman to Integer | Python | Add all curr, if curr > prev, then need to subtract 2 * prev |
| 15 | 3Sum | Python | 1. Sort and O(n^2) search with three points 2. Multiple Two Sum (Problem 1) |
| 16 | 3Sum Closest | Python | Sort and Multiple Two Sum check abs |
| 18 | 4Sum | Python | The same as 3Sum, but we can merge pairs with the same sum |
| 20 | Valid Parentheses | Python | 1. Stack 2. Replace all parentheses with '', if empty then True |
| 21 | Merge Two Sorted Lists | Python | Add a dummy head, then merge two sorted list in O(m+n) |
| 23 | Merge k Sorted Lists | Python | 1. Priority queue O(nk log k) 2. Binary merge O(nk log k) |
| 24 | Swap Nodes in Pairs | Python | Add a dummy and store the prev |
| 28 | Implement strStr() | Python | 1. O(nm) comparison 2. KMP |
| 35 | Search Insert Position | Python | Binary Search |
| 46 | Permutations | Python | 1. Python itertools.permutations 2. DFS with swapping, O(n^2) and O(n^2) 3. iteratively generate n-permutations with (n-1)-permutations, O(n^3) and O(n^2) |
| 47 | Permutations II | Python | 1. DFS with swapping, check duplicate, O(n^2) and O(n^2) 2. iteratively generate n-permutations with (n-1)-permutations, O(n^3) and O(n^2) |
| 53 | Maximum Subarray | Python | 1. Recursion, O(nlgn), O(lgn) 2. Bottom-up DP, O(n) and O(n) 3. Bottom-up DP, f(x) = max(f(x-1)+A[x], A[x]), f(x) = max(f(x-1)+A[x], A[x]),O(n) and O(1) |
| 54 | Spiral Matrix | Python | O(nm) 1. Turn in the corner and loop 2. (0, 1) -> (1, 0) -> (0, -1) -> (-1, 0) |
| 62 | Unique Paths | Python | 1. Bottom-up DP, dp[i][j] = dmap[i-1][j] + dmap[i][j-1], O(mn) and O(mn) 2. Combination (m+n-2, m-1) |
| 63 | Unique Paths II | Python | Bottom-up DP, dp[i][j] = dmap[i-1][j] + dmap[i][j-1] (if block, then 0), O(mn) and O(mn) |
| 65 | Valid Number | Python | 1. strip leading and tailing space, then check float using exception, check e using split 2. check is number split by . or e, note that number after e may be negative |
| 66 | Plus One | Python | Check if current digit == 9. |
| 70 | Climbing Stairs | Python | Bottom-up DP, dp[i] = dp[i - 2] + dp[i- 1] 1. O(n) and O(n) 2. Only two variables are needed, O(n) and O(1) |
| 72 | Edit Distance | Python | Background 1. DP O(n^2) space 2. DP O(n) space |
| 78 | Subsets | Python | 1. DFS Recursion, O(2^n) and O(2^n) 2. Recursion on a binary number, O(2^n) and O(2^n) 3. Sort and iteratively generate n subset with n-1 subset, O(n^2) and O(2^n) |
| 90 | Subsets II | Python | 1. DFS Recursion with duplicate check, O(2^n) and O(2^n) 2. Recursion on a binary number, O(2^n) and O(2^n) 3. Sort and iteratively generate n subset with n-1 subset, note that if nums[index] == nums[index - 1] then generate from last end to curr end, O(n^2) and O(2^n) |
| 94 | Binary Tree Inorder Traversal | Python | 1. Recursion, O(n) and O(1) 2. Stack and check isinstance(curr, TreeNode), O(n) and O(n) 3. Stack and check left and right, O(n) and O(n) |
| 98 | Validate Binary Search Tree | Python | 1. Stack O(n) and O(n) 2. Recursion O(n) and O(n) |
| 104 | Maximum Depth of Binary Tree | Python | Recursion max(left, right) + 1 |
| 108 | Convert Sorted Array to Binary Search Tree | Python | Recursion O(n) and O(nlgn) |
| 109 | Convert Sorted List to Binary Search Tree | Python | 1. Two points fast (next next) and slow (next) O(nlgn) and O(n) 2. Bottom-up recursion O(n) and O(lgn) |
| 110 | Balanced Binary Tree | Python | Recursion 1. Top-down O(n^2) and O(n), Bottom-up recursion with sentinel -1 O(n) and O(n) |
| 111 | Minimum Depth of Binary Tree | Python | 1. Recursion, note that when size of left (ld) or right (rd) is 0, then min = 1 + ld + rd 2. BFS check by level (right most), which is much faster than recursion |
| 124 | Binary Tree Maximum Path Sum | Python | Recursion O(n) and O(n), max (left + node, right + node, left + node + right) |
| 125 | Valid Palindrome | Python | Exclude non-alphanumeric characters and compare O(n) |
| 128 | Longest Consecutive Sequence | Python | Set or hash, pop adjacency, O(n) and O(n) |
| 133 | Clone Graph | Python | Hash and DFS or BFS |
| 136 | Single Number | Python | 1. Hash or set, O(n) and O(n) 2. xor O(n) and O(1) |
| 137 | Single Number II | Python | 1. ctypes 32 % 3 and &, O(n) and O(1) 2. ones, twos, threes as bitmask (e.g. ones represents ith bit had appeared once), O(n) and O(1) |
| 138 | Copy List with Random Pointer | Python | 1. Hash O(n) and O(n) 2. Modify original structure: Original->Copy->Original, then node.next.random = node.random.next, O(n) and O(1) |
| 141 | Linked List Cycle | Python | 1. Hash or set 2. Two points (fast and slow) 3. Add a max and check if reach the max |
| 142 | Linked List Cycle II | Python | Two points, a+b=nr |
| 143 | Reorder List | Python | 1. List as index to rebuild relation, O(n) and O(n) 2. Two points, O(n) and O(1) |
| 144 | Binary Tree Preorder Traversal | Python | 1. Recursion, O(n) and O(n) 2. Stack, O(n) and O(n) |
| 145 | Binary Tree Postorder Traversal | Python | 1. Recursion, O(n) and O(n) 2. Stack and queue (insert 0), O(n) and O(n) 3. Stack and isinstance(curr, TreeNode), O(n) and O(n) |
| 146 | LRU Cache | Python | 1. Queue and dict 2. OrderedDict |
| 150 | Evaluate Reverse Polish Notation | Python | Stack |
| 151 | Reverse Words in a String | Python | 1. Python split by space 2. Reverse all and reverse words |
| 152 | Maximum Product Subarray | Python | DP, f(k) = max(f(k-1) * A[k], A[k], g(k-1) * A[k]), g(k) = min(g(k-1) * A[k], A[k], f(k-1) * A[k]), O(n) and O(1) |
| 153 | Find Minimum in Rotated Sorted Array | Python | Binary search with conditions, A[l] > A[r] |
| 154 | Find Minimum in Rotated Sorted Array II | Python | Binary search with conditions, A[l] > A[r], A[l]=A[mid]=A[r] |
| 155 | Min Stack | Python Java | Add another stack for min stack, maintance this stack when the main stack pop or push: 1. Only push min, such that len(minStack)<=len(Stack) 2. Push min again when current top is min, such that len(minStack)=len(Stack) |
| 156 | Binary Tree Upside Down ♥ | Python | p.left = parent.right, parent.right = p.right, p.right = parent, parent = p.left, p = left |
| 157 | Read N Characters Given Read4 ♥ | Python | Handle the edge case (the end) |
| 158 | Read N Characters Given Read4 II - Call multiple times ♥ | Python | Store the pos and offset that is read by last read4 |
| 159 | Longest Substring with At Most Two Distinct Characters ♥ | Python | Maintain a sliding window that always satisfies such condition |
| 161 | One Edit Distance ♥ | Python | 1. Check the different position and conditions 2. Edit distance |
| 163 | Missing Ranges ♥ | Python | Add -1 to lower for special case, then check if curr - prev >= 2 |
| 166 | Fraction to Recurring Decimal | Python | % and Hash to find duplicate |
| 167 | Two Sum II - Input array is sorted | Python | Two points O(n) and O(1) |
| 170 | Two Sum III - Data structure design ♥ | Python | 1. Hash, O(1) for add, O(n) for find, O(n) space 2. sorted list, O(logn) for add, O(n) for find, O(n) space 3. Sort before find, O(1) for add, O(nlogn) for find, O(n) space |
| 186 | Reverse Words in a String II ♥ | Python | Reverse all and reverse each words |
| 198 | House Robber | Python | f(k) = max(f(k – 2) + num[k], f(k – 1)), O(n) and O(1) |
| 200 | Number of Islands | Python | 1. Quick union find, O(nlogn) and O(n^2) 2. BFS with marks, O(n^2) and O(1) |
| 206 | Reverse Linked List | Python | 1. Stack, O(n) and O(n) 2. Traverse on prev and curr, then curr.next = prev, O(n) and O(1) 3. Recursion, O(n) and O(1) |
| 213 | House Robber II | Python | f(k) = max(f(k – 2) + num[k], max(dp[0 |
| 216 | Combination Sum III | Python | Generate all combinations of length k and keep those that sum to n |
| 217 | Contains Duplicate | Python | 1. Set and compare length 2. Sort and check i,i +1 |
| 219 | Contains Duplicate II | Python | 1. Brute force 2. Maintenance a set that contains previous k numbers, and check if curr in set |
| 220 | Contains Duplicate III | Python | 1. Sort and binary Search 2. Bucket sort |
| 221 | Maximal Square | Python | 1. Brute force 2. dp[i][j] = min(dp[i-1][j], dp[i-1][j-1], dp[i][j-1]) + 1, O(mn) and O(mn) 3. dp[j] = min([j], dp[j-1], prev) + 1, O(mn) and O(n) |
| 228 | Summary Ranges | Python | Detect start and jump, O(n) and O(1) |
| 243 | Shortest Word Distance | Python | Update index1 and index2, and check distance, O(n) and O(1) |
| 246 | Strobogrammatic Number ♥ | Python | Hash table and reverse string, O(n) and O(n) |
| 249 | Group Shifted Strings ♥ | Python | Hash and generate hash code for each string, O(n) and O(n) |
| 252 | Meeting Rooms | Python | 1. Sort and compare intervals[i].end with intervals[i+1], O(nlogn) and O(1) 2. Sort and check intervals when count >= 2, O(nlogn) and O(n) |
| 259 | 3Sum Smaller | Python | 1. Reduce to two sum smaller, then binary search, O(n^2lgn) and O(1) 2. Reduce to two sum smaller, then two points, O(n^2) and O(1) |
| 266 | Palindrome Permutation ♥ | Python | Compute frequency, check number of odd occurrences <= 1 then palindrome, O(n) and O(n) |
| 267 | Palindrome Permutation II ♥ | Python | Check palindrome then generate half with Permutations II, O(n^2) and O(n^2) |
| 268 | Missing Number | Python Java | 1. Find missing by n * (n - 1)/2 - sum(nums) 2. XOR with index 3. Sort and binary search |
| 270 | Closest Binary Search Tree Value ♥ | Python | 1. Recursively brute force, O(n) and O(n) 2. Recursively compare root result with current kid's result (left or right), O(logn) and O(logn) 3. Iteratively compare root result with current kid's result (left or right), O(logn) and O(logn) |
| 273 | Integer to English Words | Python Java | Careful about corner cases, such 1-20 and 21-Hundred, O(lgn) and O(1) |
| 274 | H-Index | Python | Background 1. Sort and check number of papers greater than h, O(nlogn) and O(1) 2. Counting sort, O(n) and O(n) |
| 276 | Paint Fence ♥ | Python | ways[i>2] = (ways[i-1] + ways[i-2]) * (k - 1), O(n) and O(1) |
| 280 | Wiggle Sort ♥ | Python | 1. Sort and insert (n - 1) / 2 from tail to correct position, O(nlogn) and O(1) 2. Sort and swap(i, i + 1) from 1 to n - 1, O(nlogn) 3. Iteratively check order and reverse order, if not satisfied, then swap i with i + 1, O(n) |
| 286 | Walls and Gates | Python | BFS with queue, O(mn) and O(mn) |
| 288 | Unique Word Abbreviation ♥ | Python | hash |
| 293 | Flip Game ♥ | Python | Python string slicing |
| 294 | Flip Game II ♥ | Python | 1. Backtracking to ensure that next step is False, O(n!!) and O(n!!) 2. Backtracking with memo, O(n!!) and O(n!) 3. DP based on Sprague-Grundy Function |
| 296 | Best Meeting Point ♥ | Python | Think hard about Manhattan Distance in 1D case. Sort and find mean, O(mnlogmn) and O(1) |
| 298 | Binary Tree Longest Consecutive Sequence ♥ | Python | Bottom-up or top-down recursion, O(n) and O(n) |
| 305 | Number of Islands II | Python | Quick union find with weights, O(nlogn) and O(n) |
| 322 | Coin Change | Python | Bottom-up or top-down DP, dp[n] = min(dp[n], dp[n - v_i]), where v_i is the coin, O(amount * n) and O(amount) |
| 337 | House Robber III | Python | 1. Recursion with hash map, O(n) and O(n) 2. Recursion on two steps, O(n) and O(1) |
| 339 | Nested List Weight Sum ♥ | Python | Depth-first recursion |
| 340 | Longest Substring with At Most K Distinct Characters ♥ | Python | Maintain a sliding window with at most k distinct characters and a count for this window. Note that the start position need a loop to update. |
| 346 | Moving Average from Data Stream ♥ | Python | fix-sized queue or dequeue, O(1) and O(n) |
| 351 | Android Unlock Patterns ♥ | Python | Backtracking, O(n!) and O(n) |
| 359 | Logger Rate Limiter ♥ | Python | 1. hash which stores the latest timestamp, O(1) and O(n) 2. Using Priority queue to remove older logs, O(n) and O(n) |
| 366 | Find Leaves of Binary Tree ♥ | Python | 1. Set or hash to check leaft, O(n^2) and O(n) 2. Recursively check level and return them, O(n) and O(n) |
| 367 | Valid Perfect Square | Python Java | Integer square root 1. 1+3+…+(2n-1) = n^2 2. Binary search 3. Newton's method |
| 368 | Largest Divisible Subset | Python | Sort and generate x subset with previous results, O(n^2) and O(n^2) |
| 369 | Plus One Linked List ♥ | Python | 1. Stack or list that store the list, O(n) and O(n) 2. Two points, the first to the tail, the second to the latest place that is not 9, O(n) and O(1) |
| 370 | Range Addition ♥ | Python | Interval problem with cumulative sums, O(n + k) and O(n) |
| 383 | Ransom Note | Python Java | Get letter frequency (table or hash map) of magazine, then check randomNote frequency |
| 384 | Shuffle an Array | Python | Fisher–Yates shuffle, O(n) and O(n) |
| 387 | First Unique Character in a String | Python Java | Get frequency of each letter, return first letter with frequency 1, O(n) and O(1) |
| 388 | Longest Absolute File Path | Python | Store last length and rindex, O(n) and O(n) |
| 389 | Find the Difference | Python Java | 1. Imaging letter a as 0, then the sum(t)-sum(s) is the result 2. Based on solution 1, bit manipulate |
| 400 | Nth Digit | Python Java | islands * 4 - overlaps * 2 |
| 401 | Binary Watch | Python Java | Note that 12 * 60 is much less than 2^n or n^2. |
| 404 | Sum of Left Leaves | Python Java | 1. Recursively DFS with root.left.left and root.left.right check 2. The same DFS based on stack |
| 405 | Convert a Number to Hexadecimal | Python Java | Two's complement 1. Bit manipulate, each time handle 4 digits 2. Python (hex) and Java API (toHexString & Integer.toHexString) |
| 408 | Valid Word Abbreviation ♥ | Python | Go over abbr and word, O(n) and O(1) |
| 409 | Longest Palindrome | Python Java | Length of Palindrome is always 2n or 2n + 1. So, get all possible 2*n, and choose a single one as 1 if it exists. |
| 412 | Fizz Buzz | Python Java | 1. From 1 to n, condition check 2. Condition check and string connect |
| 416 | Partition Equal Subset Sum | Python | DP, Check if sum of some elements can be half of total sum, O(total_sum / 2 * n) and O(total_sum / 2) |
| 421 | Maximum XOR of Two Numbers in an Array | Python | Check 0~32 prefix, check if there is x y in prefixes, where x ^ y = answer ^ 1, O(32n) and O(n) |
| 422 | Valid Word Square ♥ | Python | Compare row with column, O(n^2) |
| 434 | Number of Segments in a String | Python Java | 1. trim &split 2. Find segment in place |
| 437 | Path Sum III | Python Java | 1. Recursively travese the whole tree, O(n^2) 2. Cache sum in Hash based on solution 1. Note that if sum(A->B)=target, then sum(root->a)-sum(root-b)=target. |
| 438 | Find All Anagrams in a String | Python Java | Build a char count list with 26-256 length. Note that this list can be update when going through the string. O(n) and O(1) |
| 443 | String Compression | Python Java | Maintain curr, read, write and anchor (start of this char). |
| 448 | Find All Numbers Disappeared in an Array | Python Java | Value (1, n) and index (0, n-1). Mark every value postion as negative. Then, the remain index with positive values are result. O(n) |
| 453 | Number of Segments in a String | Python Java | Each move is equal to minus one element in array, so the answer is the sum of all elements after minus min. |
| 461 | Hamming Distance | Python Java | Hamming Distance is related to XOR for numbers. So, XOR then count 1. O(n) |
| 463 | Island Perimeter | Python Java | math, find the area, actual number, then find the digit |
| 482 | License Key Formatting | Python Java | String processing, lower and len % K, O(n) and O(n) |
| 538 | Convert BST to Greater Tree | Python Java | Right first DFS with a variable recording sum of node.val and right.val. 1. Recursive. 2. Stack 3. Reverse Morris In-order Traversal |
| 543 | Diameter of Binary Tree | Python Java | DFS with O(1) for max answer |
| 572 | Subtree of Another Tree | Python Java | 1. Tree traverse and compare 2. Tree to string and compare |
| 581 | Shortest Unsorted Continuous Subarray | Python Java | 1. Sort and find the difference (min and max), O(nlgn) 2. Using stack to find boundaries (push when correct order, pop when not correct), O(n) and O(n) 3. Find min and max of unordered array, O(n) and O(1) |
| 617 | Merge Two Binary Trees | Python Java | Traverse both trees Recursion & Iterative (stack) |
| 654 | Maximum Binary Tree | Python Java | 1. Divide and conquer, recursive, O(n^2) 2. Monotonic stack, O(n) |
| 709 | To Lower Case | Python Java | String processing: 1. str.lower() or str.toLowerCase() 2. ASCII processing. O(n) and O(1) |
| 716 | Max Stack ♥ | Python Java | 1. Two stacks 2. Double linked list and Hash |
| 771 | Jewels and Stones | Python Java | Count given char in string. Hash or table. Oneline |
| 804 | Unique Morse Code Words | Python Java | String, Hash and Set. Set is recommended. |
| 819 | Most Common Word | Python Java | String processing, be careful about 'b,b,b'. regex is recommended. |
| 904 | Fruit Into Baskets | Python Java | 1. Scan through blocks of tree, O(n) and O(n) 2. Mainten a sliding window with start and curr point, O(n) and O(n). |
| 929 | Unique Email Addresses | Python Java | String handle and hash (or set) |
| 945 | Minimum Increment to Make Array Unique | Python Java | Sort, then list duplicate and missing value in sorted list. O(nlgn) and O(n) |
| 946 | Validate Stack Sequences | Python Java | Add a stack named inStack to help going through pushed and popped. O(n) and O(n) |
| # | To Understand |
|---|---|
| 4 | Median of Two Sorted Arrays |