遍历数组，在遍历过程中存入set，判断是否重复

import java.util.*;
public class Solution {
    public int duplicate (int[] numbers) {
        HashSet set = new HashSet();
        for(int i = 0;i<numbers.length;i++){
            if(set.contains(numbers[i])){
                return numbers[i];
            }
            set.add(numbers[i]);
        }
        return -1;
    }
}