题目：
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.
思路：
仍是递归，但是得注意(1) 必须是到 leaf node (2)Null结点的判断
package tree; public class PathSum { boolean isFirstRoot = true; public boolean hasPathSum(TreeNode root, int sum) { if (isFirstRoot && root == null) return false; isFirstRoot = false; if (root == null && sum == 0) return true; if (root == null && sum != 0) return false; if (root.left == null || root.right == null) return root.left == null ? hasPathSum(root.right, sum - root.val) : hasPathSum(root.left, sum - root.val); return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val); } }
原文：http://www.cnblogs.com/shuaiwhu/p/5118252.html