#946. Validate Stack Sequences

题目描述：

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& pushed, vector& popped) {
        int i = 0, j = 0;
        stack s;
        while (i < pushed.size() && j < popped.size()) {
            if (s.empty() || s.top() != popped[j]) {
                s.push(pushed[i]);
                i++;
            }
            else {
                s.pop();
                j++;
            }
        }
        while (!s.empty() && s.top() == popped[j]) {
            s.pop();
            j++;
        }
        return i == pushed.size() && j == popped.size();
    }
};