30. Map Sum Pairs.cpp

/*
Map Sum Pairs
=============

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.
*/

class MapSum
{
public:
  unordered_map<string, int> sums;
  unordered_map<string, int> already;

  MapSum()
  {
  }

  void insert(string key, int val)
  {
    string str = "";
    int prev = already[key];

    for (auto &i : key)
    {
      str += i;
      sums[str] += val - prev;
    }
    already[key] = val;
  }

  int sum(string prefix)
  {
    return sums[prefix];
  }
};

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