21天活动Zyf打卡第十九天Promise9913，键值映射

class MapSum:
    def __init__(self):
        self.map = {}

    def insert(self, key: str, val: int) -> None:
        self.map[key] = val

    def sum(self, prefix: str) -> int:
        res = 0
        for key,val in self.map.items():
            if key.startswith(prefix):
                res += val
        return res