PathSum.java

/*
 Author:     Annie Kim, anniekim.pku@gmail.com
 Date:       Apr 6, 2013
 Update:     Jul 26, 2013
 Problem:    Path Sum
 Difficulty: easy
 Source:     http://www.leetcode.com/onlinejudge
 Notes:
 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.
 
 Solution: Recursion.
 */
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        if (root == null) return false;
        if (root.left == null && root.right == null) return sum == root.val;
        return hasPathSum(root.left, sum - root.val) || 
               hasPathSum(root.right, sum - root.val);
    }
}