112. Path Sum

Problem:

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.

Analysis:

From top to bottom, subtract sum by root.val. Once reach bottom, check whether val minus remaining sum == 0.

Solution:

 /**  
  * Definition for a binary tree node.  
  * public class TreeNode {  
  *   int val;  
  *   TreeNode left;  
  *   TreeNode right;  
  *   TreeNode(int x) { val = x; }  
  * }  
  */  
 class Solution {  
   public boolean hasPathSum(TreeNode root, int sum) {  
     if (root == null) return false;  
     if (root.left == null && root.right == null && root.val - sum == 0) return true;  
     return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val);  
   }  
 }