Method insert

677-map-sum-pairs.rs:20–26

(&mut self, key: String, val: i32)

    }

    fn insert(&mut self, key: String, val: i32) {
        if let Some(x) = self.mmp.get_mut(&key) {
            *x = val;
        } else {
            self.mmp.insert(key, val);
        }
    }

    fn sum(&mut self, prefix: String) -> i32 {
        let mut ret = 0;