677. Map Sum Pairs 配对之和

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

使用insert和sum方法实现MapSum类。 对于插入方法，您将得到一对（字符串，整数）。字符串表示键，整数表示该值。如果密钥已经存在，那么原来的密钥对对将被覆盖。 对于方法总和，您将得到一个表示前缀的字符串，您需要返回所有键的值以前缀开头的值的总和。

/**
 * Initialize your data structure here.
 */
var MapSum = function() {
  this.map = new Map();
};
/** 
* @param {string} key 
* @param {number} val
* @return {void}
*/
MapSum.prototype.insert = function (key, val) {
  this.map.set(key, val);
};
/** 
* @param {string} prefix
* @return {number}
*/
MapSum.prototype.sum = function (prefix) {
  let times = 0;
  let map = this.map;
  for (let item of map) { 
    if (item[0].slice(0,prefix.length) == (prefix)) { 
      times += item[1];
    }
  }
  return times;
};
/** 
 * Your MapSum object will be instantiated and called as such:
 * var obj = Object.create(MapSum).createNew()
 * obj.insert(key,val)
 * var param_2 = obj.sum(prefix)
 */

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