Tags: "leetcode", "stack", "greedy", access_time 1-min read

Validate Stack Sequence

Created: March 22, 2020 by [lek-tin]

Last updated: March 22, 2020

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

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

        for num in pushed:
            stack.append(num)

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

        return j == len(popped)