Build an Array With Stack Operations

Problem Statement - link #

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.

Examples #

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 #

class Solution {
public:
    vector<string> buildArray(vector<int>& target, int n) {
        vector<string> ans;
        for(int i=0, cur=1; i<target.size(); ++cur)
            if(target[i] == cur) ans.push_back("Push"), ++i;
            else ans.push_back("Push"), ans.push_back("Pop");
        return ans;
    }
};