Given a binary tree and a sum, return true if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum. Return false if no such path can be found.

We'll define a "root-to-leaf path" to be a sequence of nodes in a tree starting with the root node and proceeding downward to a leaf (a node with no children). We'll say that an empty tree contains no root-to-leaf paths. So for example, the following tree has exactly four root-to-leaf paths:

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \      \
        7    2      1

Root-to-leaf paths:
   path 1: 5 4 11 7
   path 2: 5 4 11 2
   path 3: 5 8 13
   path 4: 5 8 4 1

For this problem, we will be concerned with the sum of the values of such a path -- for example, the sum of the values on the 5-4-11-7 path is 5 + 4 + 11 + 7 = 27.

subtract the node value from the sum when recurring down, and check to see if the sum is 0 when you run out of tree.