946. Validate Stack Sequences.md

946. Validate Stack Sequences

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.

Constraints:

• 0 <= pushed.length == popped.length <= 1000
• 0 <= pushed[i], popped[i] < 1000
• pushed is a permutation of popped.
• pushed and popped have distinct values.

代码

class Solution {
public:
    bool validateStackSequences(vector<int>& pushed, vector<int>& popped) {
        stack<int> s;
        int i = 0;
        for (int x : pushed) {
            s.push(x);
            while (!s.empty() && s.top() == popped[i]) {
                s.pop();
                i++;
            }
        }
        return s.empty();
    }
};

class Solution:
    def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
        
        stack = []
        for i in pushed:
            stack.append(i)
            while stack and popped and stack[-1] == popped[0]:
                # stack top
                stack.pop()
                popped.pop(0)
                
        return not stack