LeetCode-in-Java

1441. Build an Array With Stack Operations

Easy

You are 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": Reads a new element from the beginning list, and pushes it in the array.
• "Pop": Deletes the last element of the array.
• If the target array is already built, stop reading more elements.

Return a list of the operations needed to build target. The test cases are generated so 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.

Constraints:

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

Solution

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class Solution {
    public List<String> buildArray(int[] target, int n) {
        List<String> result = new ArrayList<>();
        Set<Integer> set = new HashSet<>();
        for (int i : target) {
            set.add(i);
        }
        int max = target[target.length - 1];
        for (int i = 1; i <= n; i++) {
            if (!set.contains(i)) {
                result.add("Push");
                result.add("Pop");
            } else {
                result.add("Push");
            }
            if (i == max) {
                break;
            }
        }
        return result;
    }
}

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