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:

  1. 0 <= pushed.length == popped.length <= 1000
  2. 0 <= pushed[i], popped[i] < 1000
  3. pushed is a permutation of popped.
  4. pushed and popped have distinct values.

Solution

public class Solution {
    public bool ValidateStackSequences(int[] pushed, int[] popped) {
        Stack<int> s = new Stack<int>();
        
        int j = 0;
        foreach(int num in pushed){
            s.Push(num);
            
            while(s.Count > 0 && 
                  j < pushed.Length && 
                  s.Peek() == popped[j]){
                s.Pop();
                j++;
            }
        }
        
        return j == popped.Length;
    }
}

Time Complexity: O(n)

Space Complexity: O(n)

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