先对[0,n] 排序，然后从1开始，依次比较当前数字nums[i]和前一个数字nums[i-1]是否相等,如果相等，就返回当前值，即为重复数字

 Given an array nums containing n + 1 integers where each integer is between 1 and n (inclusive), prove that at least one duplicate number must exist. Assume that there is only one duplicate number, find the duplicate one.

Note:

    You must not modify the array (assume the array is read only).
    You must use only constant, O(1) extra space.
    Your runtime complexity should be less than O(n2).
    There is only one duplicate number in the array, but it could be repeated more than once.




    int cmp ( const  void *a , const  void *b )    
    {    
        return *(int *)a - *(int *)b;    
    }

    int findDuplicate(int* nums, int numsSize) {  
        int i;  
        qsort(nums,numsSize,sizeof(int),cmp);  
          
        for(i=1;i<numsSize;i++)  
        {  
            if(nums[i-1]==nums[i])  
                return nums[i];  
        }
        
        return -1;
    }