Graphs, Heaps and Tree

DFS
To make sure the depth-first search algorithm doesn't re-visit vertices, the visited HashSet keeps track of vertices already visited. A Stack, called stack, keeps track of vertices found but not yet visited. Initially stack contains the starting vertex. The next vertex is popped from stack. If it has already been visited, it is discarded and the next vertex is popped from stack. If it has not been visited, it is added to the set of visited vertices and its unvisited neighbors are added to stack. This continues until there are no more vertices in stack, and the set of vertices visited is returned. The set of vertices returned is all the vertices that can be reached from the starting vertex

Few Good Ones

Graph

• Clone Graph - https://leetcode.com/problems/clone-graph/
• Course Schedule - https://leetcode.com/problems/course-schedule/
• Pacific Atlantic Water Flow - https://leetcode.com/problems/pacific-atlantic-water-flow/
• Number of Islands - https://leetcode.com/problems/number-of-islands/
• Longest Consecutive Sequence - https://leetcode.com/problems/longest-consecutive-sequence/
• Alien Dictionary (Leetcode Premium) - https://leetcode.com/problems/alien-dictionary/
• Graph Valid Tree (Leetcode Premium) - https://leetcode.com/problems/graph-valid-tree/
• Number of Connected Components in an Undirected Graph (Leetcode Premium) - https://leetcode.com/problems/number-of-connected-components-in-an-undirected-graph/

Tree

• Maximum Depth of Binary Tree - https://leetcode.com/problems/maximum-depth-of-binary-tree/
• Same Tree - https://leetcode.com/problems/same-tree/
• Invert/Flip Binary Tree - https://leetcode.com/problems/invert-binary-tree/
• Binary Tree Maximum Path Sum - https://leetcode.com/problems/binary-tree-maximum-path-sum/
• Binary Tree Level Order Traversal - https://leetcode.com/problems/binary-tree-level-order-traversal/
• Serialize and Deserialize Binary Tree - https://leetcode.com/problems/serialize-and-deserialize-binary-tree/
• Subtree of Another Tree - https://leetcode.com/problems/subtree-of-another-tree/
• Construct Binary Tree from Preorder and Inorder Traversal - https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
• Validate Binary Search Tree - https://leetcode.com/problems/validate-binary-search-tree/
• Kth Smallest Element in a BST - https://leetcode.com/problems/kth-smallest-element-in-a-bst/
• Lowest Common Ancestor of BST - https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/
• Implement Trie (Prefix Tree) - https://leetcode.com/problems/implement-trie-prefix-tree/
• Add and Search Word - https://leetcode.com/problems/add-and-search-word-data-structure-design/
• Word Search II - https://leetcode.com/problems/word-search-ii/