13. 找出数组中重复的数字

class Solution {
public:
    int duplicateInArray(vector<int>& nums) {
        map<int, bool> box;
        for (auto v : nums) 
            if (v < 0 || v >= nums.size()) 
                return -1;
        for (auto v : nums)
            if (box[v]) return v;
            else box[v] = 1;
        return -1;
    }
};