946-validate-stack-sequences.py

"""
Problem Link: https://leetcode.com/problems/validate-stack-sequences/

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:
0 <= pushed.length == popped.length <= 1000
0 <= pushed[i], popped[i] < 1000
pushed is a permutation of popped.
pushed and popped have distinct values.
"""
class Solution(object):
    def validateStackSequences(self, pushed, popped):
        """
        :type pushed: List[int]
        :type popped: List[int]
        :rtype: bool
        """
        idx = 0
        stack = []
        l = len(popped)
        for i in pushed:
            stack.append(i)
            while stack and stack[-1] == popped[idx]:
                stack.pop()
                idx += 1
        return idx == l