LeetCode 112 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.
需要注意仅有叶子节点开始的，才是符合要求的路径。因为仅当left和right为null时，才判断sum与当前值是否相等；若仅有一个left || right为null，则继续判断不为null的一侧。
public class Solution {
public boolean hasPathSum(TreeNode root, int sum) {
if (root == null) {
return false;
} else {
if (root.left == null && root.right == null) {
if (root.val == sum) return true;
else return false;
} else {
return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val);
}
}
}
}
