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.

思路：就是pushed直接push进stack，遇见poped，就pop，然后看是否能够最后是空stack。

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