README_EN.md

1441. Build an Array With Stack Operations

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Description

Given an array target and an integer n. In each iteration, you will read a number from  list = {1,2,3..., n}.

Build the target array using the following operations:

• Push: Read a new element from the beginning list, and push it in the array.
• Pop: delete the last element of the array.
• If the target array is already built, stop reading more elements.

Return the operations to build the target array. You are guaranteed that the answer is unique.

 

Example 1:

Input: target = [1,3], n = 3
Output: ["Push","Push","Pop","Push"]
Explanation: 
Read number 1 and automatically push in the array -> [1]
Read number 2 and automatically push in the array then Pop it -> [1]
Read number 3 and automatically push in the array -> [1,3]

Example 2:

Input: target = [1,2,3], n = 3
Output: ["Push","Push","Push"]

Example 3:

Input: target = [1,2], n = 4
Output: ["Push","Push"]
Explanation: You only need to read the first 2 numbers and stop.

Example 4:

Input: target = [2,3,4], n = 4
Output: ["Push","Pop","Push","Push","Push"]

 

Constraints:

• 1 <= target.length <= 100
• 1 <= target[i] <= n
• 1 <= n <= 100
• target is strictly increasing.

Solutions

Python3

class Solution:
    def buildArray(self, target: List[int], n: int) -> List[str]:
        cur, ans = 1, []
        for t in target:
            for i in range(cur, n + 1):
                ans.append('Push')
                if t == i:
                    cur = i + 1
                    break
                ans.append('Pop')
        return ans

Java

class Solution {
    public List<String> buildArray(int[] target, int n) {
        List<String> ans = new ArrayList<>();
        int cur = 1;
        for (int t : target) {
            for (int i = cur; i <= n; ++i) {
                ans.add("Push");
                if (t == i) {
                    cur = i + 1;
                    break;
                }
                ans.add("Pop");
            }
        }
        return ans;
    }
}

C++

class Solution {
public:
    vector<string> buildArray(vector<int>& target, int n) {
        vector<string> ans;
        int cur = 1;
        for (int t : target)
        {
            for (int i = cur; i <= n; ++i)
            {
                ans.push_back("Push");
                if (t == i)
                {
                    cur = i + 1;
                    break;
                }
                ans.push_back("Pop");
            }
        }
        return ans;
    }
};

Go

func buildArray(target []int, n int) []string {
	var ans []string
	cur := 1
	for _, t := range target {
		for i := cur; i <= n; i++ {
			ans = append(ans, "Push")
			if t == i {
				cur = i + 1
				break
			}
			ans = append(ans, "Pop")
		}
	}
	return ans
}

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