MapSum

All Implemented Interfaces:
```
public final class MapSum

                    
```
677 - Map Sum Pairs.
Medium
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.

Nested Class Summary

Nested Classes
Modifier and Type	Class	Description

Field Summary

Fields
Modifier and Type	Field	Description

Constructor Summary

Constructors
Constructor	Description
`MapSum()`

Enum Constant Summary

Enum Constants
Enum Constant	Description

Method Summary

Modifier and Type	Method	Description
`final Unit`	`insert(String key, Integer val)`
`final Integer`	`sum(String prefix)`

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail
- MapSum
```
MapSum()
```

Method Detail

insert

 final Unit insert(String key, Integer val)

sum
```
 final Integer sum(String prefix)
```