Path Sum

Given a binary tree and a sum, determine if the tree has a root-to-leaf
path such that adding up all the values along the path equals the given
sum.

For example:
Given the below binary tree and sum = 22,
              5
             / \
            4   8
           /   / \
          11  13  4
         /  \      \
        7    2      1
return true, as there exist a root-to-leaf path 5->4->11->2 which sum is
22.