[leetcode]946. Validate Stack Sequences用Stack验证序列

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.

题意

给定两个序列push[]、popped[]。 用Stack验证用两个序列的元素，经过一番push和pop操作， Stack能为空

Solution： Stack

code

class Solution {
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        Stack<Integer> stack = new Stack<>();
        int i = 0;
        for (int p : pushed) {
            stack.push(p);
            while (!stack.isEmpty() && stack.peek() == popped[i]) {
                stack.pop();
                ++i;
            }
        }
        return stack.empty();
    }
}