問題描述

 946. 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.

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.

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題意

給定兩個數列pushed和popped，判斷pushed和popped是否可以按順序模擬入棧出棧過程。

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思路

用棧模擬操作。每次入棧一個pushed的一個數後儘量多的出棧，如果最後棧空，則返回true

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程式碼

class Solution {
public:
    bool validateStackSequences(vector<int>& pushed, vector<int>& popped) {
        int i = 0, j = 0, k = -1, n = pushed.size();
        vector<int> st;
        while (i < n)
        {
            st.push_back(pushed[i++]);
            k++;
            while (j < n && k >= 0)
            {
                if (st[k] == popped[j])
                {
                    st.pop_back();
                    k--;
                    j++;
                }
                else
                {
                    break;
                }
            }
        }
        return (k == -1);
    }
};