Validate Stack Sequences

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:

class Solution {
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        int N = pushed.length;
        Stack<Integer> stack = new Stack();

        int j = 0;
        for (int x: pushed) {
            stack.push(x);
            while (!stack.isEmpty() && j < N && stack.peek() == popped[j]) {
                stack.pop();
                j++;
            }
        }

        return j == N;
    }
}
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