Java代码

class Solution {
    public int duplicateInArray(int[] nums) {
        if(nums == null || nums.length < 2)
            return -1;
        int low = 1;
        int high = nums.length - 1;
        int mid = 0, res = 0;

        while(low < high) {
            mid = low + (high - low) / 2;
            res = count(nums, low, mid); //统计[low,mid]之间的元素个数

            if(res > mid - low + 1) { //说明[low,mid]之间有重复数字
                high = mid;
            }
            else { //否则说明[mid+1,high]之间有重复数字
                low = mid + 1;
            }
        }
        return low;
    }

    //统计nums中，元素值在[low,high]范围内元素的个数
    public int count(int[] nums, int low, int high) {
        int ans = 0;
        for (int i : nums) {
            if(i >= low && i <= high)
                ans ++;
        }
        return ans;
    }
}