source-code/Contains_Duplicate.cpp

class Solution {
public:
    bool containsDuplicate(vector<int>& nums) {
        unordered_map <int, bool> exist;
        for(int i = 0; i < nums.size(); ++i) {
            if(exist[nums[i]]) {
                return true;
            }
            exist[nums[i]] = true;
        }
        return false;
    }
};