946.Validate Stack Sequence(验证栈序列)

Description

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.

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给定 pushed 和 popped 两个序列，只有当它们可能是在最初空栈上进行的推入 push 和弹出 pop 操作序列的结果时，返回 true；否则，返回 false 。

题目链接：https://leetcode.com/problems/validate-stack-sequences/

Difficulty: medium

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.

分析

1. 模拟栈的操作，以此来匹配进栈序列和出栈序列；
2. 若栈为空或者栈顶元素不等于当前出栈数字：将进栈数字压入栈（若没有进栈数字了，返回False），循环2；
3. pop出栈顶元素；
4. 遍历完所有出栈数字，返回True。

参考代码

class Solution:
def validateStackSequences(self, pushed, popped):
    i=0
    stack=[]
    for pop in popped:
        while(len(stack)==0 or stack[-1]!=pop):
            if(i<len(pushed)):
                stack.append(pushed[i])
                i+=1
            else:
                return False
        stack=stack[:-1]
    return True