# 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.

• 它要求一定要到left.
• 然后空节点不能当成0.