# 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.

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

Github 同步地址:

https://github.com/grandyang/leetcode/issues/946

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

https://leetcode.com/problems/validate-stack-sequences/discuss/197685/C%2B%2BJavaPython-Simulation-O(1)-Space

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