LeetCode 946. Validate Stack Sequences

原题链接在这里：https://leetcode.com/problems/validate-stack-sequences/

题目：

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.

题解：

Mimic the operation of stack, push the current pushed element.

And while stk peek == current popped element, keep poping.

Go through each of the pushed and check eventually if the stack is empty.

Time Complexity: O(n). n = pushed.length.

Space: O(n).

AC Java:

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