LeetCode - Stack - Build an Array With Stack Operations

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.

You are guaranteed that the target array is strictly increasing, only containing numbers between 1 to n inclusive.

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"]

 

class Solution {
    public List<String> buildArray(int[] target, int n) {
        int i=0;
        int j=1;
        List<String> stackOps = new ArrayList<String>();
        while(j<=n){
            if(i == target.length) break;
            if(target[i] == j){
               stackOps.add("Push");
               i++;
            }else{
                stackOps.add("Push");
                stackOps.add("Pop");
            }
            j++;
        }
        return stackOps;
    }
}

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