Validate Stack Sequences

LeetCode Q 946 - Validate Stack Sequences

Given two sequences pushed and popped with distinct values, return true if and only if this could have been the result of a sequence of push and pop operations on an initially empty stack.

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.

Note:

• 0 <= pushed.length == popped.length <= 1000
• 0 <= pushed[i], popped[i] < 1000
• pushed is a permutation of popped.
• pushed and popped have distinct values.

Solution

Code

public boolean validateStackSequences(int[] pushed, int[] popped) {
  if (pushed.length == 0)
    return true;

  Stack<Integer> stack = new Stack<>();
  
  int popIdx = 0;

  for (int i = 0; i < pushed.length; i++) {
    stack.push(pushed[i]);
    
    while ( !stack.isEmpty() && stack.peek() == popped[popIdx]) {
      stack.pop(); popIdx++;  
    }
  }

  return stack.isEmpty();
}