java实现

//方法一：稍微快点

class Solution {
        public int duplicateInArray(int[] nums) {
        if(nums.length==0)return -1;
        Arrays.sort(nums);
        if(nums[0]<0||nums[nums.length-1]>nums.length-1)
            return -1;
        for (int i = 0; i < nums.length-1; i++) {
            if(nums[i]==nums[i+1]){
                return nums[i];
            }
        }
        return -1;
    }
}

//方法二：

public static  int duplicateInArray(int[] nums) {
        Set<Integer>set=new HashSet<>();
        for (int i = 0; i < nums.length; i++) {
            if(nums[i]<0||nums[i]>nums.length-1)return -1;
        }
        for (int i = 0; i < nums.length; i++) {
            if(set.contains(nums[i]))return nums[i];
            set.add(nums[i]);
        }
        return -1;
    }