Sword finger offer: push in and pop-up sequence of stack

subject

Enter two sequences of integers , The first sequence represents the push order of the stack , Determine whether the second sequence is the pop-up order for the stack . Suppose that all the Numbers pushed are not equal . for example , Sequence {1,2,3,4,5} Is the stack pressing sequence of a stack , Sequence {4,5,3,2,1} Is a popup sequence corresponding to the stack sequence , but {4,3,5,1,2} It cannot be a popup sequence of the push sequence .

Example

Input ：pushed = [1,2,3,4,5], popped = [4,5,3,2,1]
Output ：true
explain ： We can do it in the following order ：
push(1), push(2), push(3), push(4), pop() -> 4,
push(5), pop() -> 5, pop() -> 3, pop() -> 2, pop() -> 1

Input ：pushed = [1,2,3,4,5], popped = [4,3,5,1,2]
Output ：false
explain ：1 Can't be in 2 Pop up before .

Code

Borrow an auxiliary stack stackstack , simulation Push the / Arrangement of pop-up operations . Depending on whether the simulation is successful , You get the result .
Into the stack, ： Execute in the order of stack pressing sequence .
The stack, ： After each stack , Circular judgement “ Top element of stack == Pop up the current element of the sequence ” Is it true , Pop up all stack top elements that meet the pop-up sequence order .
Answer key

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

    }
}