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.

Solution

Time complexity : O(n)
Space complexity : O(n)

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

模擬依序 push，若可 pop 則 pop。
最後檢查該 stack 是否全被 pop 掉，是的話表示該 pop 順序是合理的。