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 <code>sum = 22</code>,
return true, as there exist a root-to-leaf
path <code>5->4->11->2</code> which sum is 22.
递归求解就可以。注意叶子结点条件是左右结点都为空。
given a binary tree and a sum, find all root-to-leaf paths where each path‘s
sum equals the given sum.
return