677. Map Sum Pairs

Implement a MapSum class with insert, and sum methods.

For the method insert, you'll be given a pair of (string, integer). The string represents the key and the integer represents the value. If the key already existed, then the original key-value pair will be overridden to the new one.

For the method sum, you'll be given a string representing the prefix, and you need to return the sum of all the pairs' value whose key starts with the prefix.

Example 1:

Input: insert("apple", 3), Output: NullInput: sum("ap"), Output: 3Input: insert("app", 2), Output: NullInput: sum("ap"), Output: 5

实现MapSum 数据结构，其中包含两个方法insert和sum。insert是记录一个字符串-整数对。sum是求所有键值的前缀为prefix的对的数字和。使用unordered_map记录键值对。实现starts_with函数来判断是否是前缀。则insert和sum方法很容易就实现了。

代码：

class MapSum {public:    /** Initialize your data structure here. */    MapSum() {            }        void insert(string key, int val) {        pairs[key] = val;    }        int sum(string prefix) {        int res = 0;        for(auto p:pairs) {            if(starts_with(p.first, prefix)) {                res += p.second;            }        }        return res;    }private:    unordered_map<string, int> pairs;    bool starts_with(string s, string t) {        return s.substr(0, t.size()) == t;    }};