# 946. 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.

# Solution

Approach 1: simulate with a stack -- O(N) time and space.

# Code (Python)

Approach 1:

class Solution:
    def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:        
        stack = []
        stack_elements = set()
        next_to_push = 0
        for num in popped:
            if num in stack_elements:
                if stack[-1] != num:
                    return False
                else:
                    stack_elements.remove(stack[-1])
                    stack.pop()
            else:
                while pushed[next_to_push] != num:
                    stack.append(pushed[next_to_push])
                    stack_elements.add(pushed[next_to_push])
                    next_to_push += 1
                next_to_push += 1
        return True

# Code (C++)

Approach 1:

Approach 2: