数组中重复的数字

class Solution {
public:
    // Parameters:
    //        numbers:     an array of integers
    //        length:      the length of array numbers
    //        duplication: (Output) the duplicated number in the array number
    // Return value:       true if the input is valid, and there are some duplications in the array number
    //                     otherwise false
     bool duplicate(int numbers[], int length, int* duplication) {
        bool flag[length];
        memset(flag,0,sizeof(int)*length);
        for(int i = 0; i < length;i++)
        {
           // if(numbers[i] > length - 1 || numbers[i] < 0)
             //   return false;
            if(flag[numbers[i]] == true)
            {
                *duplication = numbers[i];
                return true;
            }else
                flag[numbers[i]] = true;
        }
        return false;
    }
};

    bool duplicate(int numbers[], int length, int* duplication) {
        for ( int i= 0 ; i<length; i++) {
            int index = numbers[i];
            if (index >= length) {//如果该数字被改过，则-length取得原来的值
                index -= length;
            }   
            if (numbers[index] >= length) { //对应数字被改过，说明该数字出现过
                *duplication = index;
                return true;
            }   
            numbers[index] = numbers[index] + length;
        }
        return false;
    }