【剑指offer】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) 
    {
        set<int> s;    //用来记录数组中出现过的数字
        for(int i = 0; i < length; i++)    
        {
            if(s.count(numbers[i]) == 0)    //若该数字没在数组中出现过
            {
                s.insert(numbers[i]);
            }
            else     //若该数字在数组中已经出现过了
            {
                *duplication = numbers[i];    //numbers[i]就是数组中第一个重复的数字
                return true;
            }
        }   
        return false;
    }
};