题目

原题在此
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.

解析

模拟栈的操作, 将pushed依次入栈, 并在每次入栈后尝试pop尽可能多的元素. 最后若popped完全遍历则是合法的.

代码

c++

class Solution {
public:
    bool validateStackSequences(vector<int>& pushed, vector<int>& popped) {
        stack<int> s;
        auto it = popped.begin();
        for(int i : pushed) {
            s.push(i);
            while(!s.empty() && *it == s.top()) {
                it++;
                s.pop();
            }
        }
        return it == popped.end();
    }
};