text: class Solution:
      # @return a tuple, (index1, index2)
      def twoSum(self, num, target):
          a = dict()
          for i,n in enumerate(num):
              if target - n in a:
                  return (a[target-n],i+1)
              if n not in a:
                  a[n] = i+1
          return None
  