677. Map Sum Pairs

题目

Design a map that allows you to do the following:

Maps a string key to a given value.
Returns the sum of the values that have a key with a prefix equal to a given string.

Implement the MapSum class:

MapSum() Initializes the MapSum object.
void insert(String key, int val) Inserts the key-val pair into the map. If the key already existed, the original key-value pair will be overridden to the new one.
int sum(string prefix) Returns the sum of all the pairs' value whose key starts with the prefix.

Example 1:

Input
["MapSum", "insert", "sum", "insert", "sum"]
[[], ["apple", 3], ["ap"], ["app", 2], ["ap"]]
Output
[null, null, 3, null, 5]

Explanation
MapSum mapSum = new MapSum();
mapSum.insert("apple", 3);
mapSum.sum("ap");           // return 3 (apple = 3)
mapSum.insert("app", 2);
mapSum.sum("ap");           // return 5 (apple +app = 3 + 2 = 5)

Constraints:

1 <= key.length, prefix.length <= 50
key and prefix consist of only lowercase English letters.
1 <= val <= 1000
At most 50 calls will be made to insert and sum.

题目大意

实现一个 MapSum 类，支持两个方法，insert 和 sum：

MapSum() 初始化 MapSum 对象
void insert(String key, int val) 插入 key-val 键值对，字符串表示键 key ，整数表示值 val 。如果键 key 已经存在，那么原来的键值对将被替代成新的键值对。
int sum(string prefix) 返回所有以该前缀 prefix 开头的键 key 的值的总和。

解题思路

简单题。用一个 map 存储数据，Insert() 方法即存储 key-value。Sum() 方法即累加满足条件前缀对应的 value。判断是否满足条件，先根据前缀长度来判断，只有长度大于等于 prefix 长度才可能满足要求。如果 key 是具有 prefix 前缀的，那么累加上这个值。最后输出总和即可。

代码

package leetcode

type MapSum struct {
	keys map[string]int
}

/** Initialize your data structure here. */
func Constructor() MapSum {
	return MapSum{make(map[string]int)}
}

func (this *MapSum) Insert(key string, val int) {
	this.keys[key] = val
}

func (this *MapSum) Sum(prefix string) int {
	prefixAsRunes, res := []rune(prefix), 0
	for key, val := range this.keys {
		if len(key) >= len(prefix) {
			shouldSum := true
			for i, char := range key {
				if i >= len(prefixAsRunes) {
					break
				}
				if prefixAsRunes[i] != char {
					shouldSum = false
					break
				}
			}
			if shouldSum {
				res += val
			}
		}
	}
	return res
}

/**
 * Your MapSum object will be instantiated and called as such:
 * obj := Constructor();
 * obj.Insert(key,val);
 * param_2 := obj.Sum(prefix);
 */