twoSum（C++）

vector<int> twoSum(vector<int> &numbers, int target)
{
    //Key is the number and value is its index in the vector.
    unordered_map<int, int> hash;  //unordered_map是无序关联容器
    vector<int> result;
    for (int i = 0; i < numbers.size(); i++) {
        int numberToFind = target - numbers[i];

            //if numberToFind is found in map, return them
        if (hash.find(numberToFind) != hash.end()) {  //find（）返回一个迭代器，指向区间[first,last]（hash的序号）中第一个值为value的元素，如果没有找到就返回last
                    //+1 because indices are NOT zero based
            result.push_back(hash[numberToFind] + 1);//push_back(t)将t插到a.end()前面
            result.push_back(i + 1);            
            return result;
        }

            //number was not found. Put it in the map.
        hash[numbers[i]] = i;
    }
    return result;
}

无序关联容器有：unordered_set     unordered_multiset      unordered_map        unordered_multimap,它们使用键和哈希表，以便能够快速存取数据。没有了有序关联容器的lower_bound 和upper_bound，所比较基于等于概念。