[Google Question][LeetCode] Validate Stack Sequences

Problem: 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:

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

Input: pushed = [1,2,3,4,5], popped = [4,3,5,1,2]
Output: false
Explanation: 1 cannot be popped before 2.

Approach: We can use stack itself :) to see if the sequence of pushes and pops are valid or not. Once we use stack, the approach is straight forward. It looks like:

• popIndex = 0
• FOR i = 0 to n - 1
• Stack.Push(pushed[i])
• WHILE Stack not empty AND Stack.Top == popped[popIndex]
• Stack.Pop()
• popIndex = popIndex + 1
• RETURN popIndex == n

That's all!

Implementation in C#:

    public bool ValidateStackSequences(int[] pushed, int[] popped) 

    {

        int j = 0;

        Stack<int> stack = new Stack<int>();

        foreach (int num in pushed)

        {

            stack.Push(num);

            while (stack.Count > 0 && stack.Peek() == popped[j])

            {

                stack.Pop();

                ++j;

            }

        }

        return j == popped.Length;

    }

Complexity: O(n)