946. Validate Stack Sequences

Description

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

validate-stack-sequences.py

class Solution:
    def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
        n = len(pushed)
        stack = []
        curr = 0

        for x in pushed:
            stack.append(x)

            while stack and curr < n and stack[-1] == popped[curr]:
                stack.pop()
                curr += 1

        return len(stack) == 0 and curr == n