LeetCode-in-Java

946. Validate Stack Sequences

Medium

Given two integer arrays pushed and popped each with distinct values, return true if this could have been the result of a sequence of push and pop operations on an initially empty stack, or false otherwise.

Example 1:

Input: pushed = [1,2,3,4,5], popped = [4,5,3,2,1]

Output: true

Explanation: We might do the following sequence:

push(1), push(2), push(3), push(4),

pop() -> 4,

push(5),

pop() -> 5, pop() -> 3, pop() -> 2, pop() -> 1

Example 2:

Input: pushed = [1,2,3,4,5], popped = [4,3,5,1,2]

Output: false

Explanation: 1 cannot be popped before 2.

Constraints:

• 1 <= pushed.length <= 1000
• 0 <= pushed[i] <= 1000
• All the elements of pushed are unique.
• popped.length == pushed.length
• popped is a permutation of pushed.

Solution

import java.util.ArrayDeque;
import java.util.Deque;

public class Solution {
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        Deque<Integer> stack = new ArrayDeque<>();
        int i = 0;
        int j = 0;
        int len = pushed.length;
        while (i < len) {
            if (pushed[i] == popped[j]) {
                i++;
                j++;
            } else if (!stack.isEmpty() && stack.peek() == popped[j]) {
                stack.pop();
                j++;
            } else {
                stack.push(pushed[i++]);
            }
        }
        while (j < len) {
            if (!stack.isEmpty() && stack.peek() != popped[j++]) {
                return false;
            } else {
                stack.pop();
            }
        }
        return true;
    }
}

This site is open source. Improve this page.