一. 思路

有两种解决方法：

• 将数组排序，相邻两个数字相同则有重复；
• 采用HashSet，遍历数组，set中不包含重复数字，则将数字放进set中

二. 采用哈希方法解决的代码

import java.util.*;
public class Solution {
    // Parameters:
    //    numbers:     an array of integers
    //    length:      the length of array numbers
    //    duplication: (Output) the duplicated number in the array number,length of duplication array is 1,so using duplication[0] = ? in implementation;
    //                  Here duplication like pointor in C/C++, duplication[0] equal *duplication in C/C++
    //    这里要特别注意~返回任意重复的一个，赋值duplication[0]
    // Return value:       true if the input is valid, and there are some duplications in the array number
    //                     otherwise false
    public boolean duplicate(int numbers[],int length,int [] duplication) {
        Set<Integer> set = new HashSet<>();

        for (int i = 0; i < length; i++) {
            if (set.contains(numbers[i])) {
                duplication[0] = numbers[i];
                return true;
            }
            set.add(numbers[i]);
        }

        return false;
    }
}