946.Validate Stack Sequences

Tags: Medium Stack

Links: https://leetcode.com/problems/validate-stack-sequences/

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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.

---

class Solution {
public:
    bool validateStackSequences(vector<int>& pushed, vector<int>& popped) {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        stack<int> s;
        int pos1 = 0, pos2 = 0;
        int n = pushed.size(); if ((int)popped.size() != n) return false;

        while (pos1 < n || pos2 < n) {
            if (s.empty()) {
                s.push(pushed[pos1++]);
            }
            else {
                if (s.top() == popped[pos2]) {
                    s.pop(); ++pos2;
                }
                else if (pos1 >= n) break;
                else s.push(pushed[pos1++]);
            }
        }

        return s.empty();
    }
};

这道题目可以和一本通 1357对比，其实是一个道理。另外如果在1357的基础上扩展，栈的容量是有限的，那么就是SJTU OJ 的1049 火车调度了。