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

思路：

For each key in the map, if that key starts with the given prefix, then add it to the answer.

class MapSum {
HashMap<String, Integer> map;
/** Initialize your data structure here. */
public MapSum() {
map = new HashMap<>();
}

public void insert(String key, int val) {
map.put(key, val);
}

public int sum(String prefix) {
int ans = 0;
for (String key: map.keySet()) {
if (key.startsWith(prefix)) {
ans += map.get(key);
}
}
return ans;
}
}

/**
* Your MapSum object will be instantiated and called as such:
* MapSum obj = new MapSum();
* obj.insert(key,val);
* int param_2 = obj.sum(prefix);
*/