946. Validate Stack Sequences

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題目敘述

Given two integer arrays pushed and popped each with distinct values, return true if this could have been the result of a sequence of push and pop operations on an initially empty stack, or false otherwise.

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.

解題思路

Solution

import java.util.Stack;

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