message: Update map-sum-pairs.py
diff: # Time: O(n), n is the length of key
  # Space: O(t), t is the number of nodes in trie
  +# 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
  +
  class MapSum(object):
  def __init__(self):
  