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.

hasPathSum.java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 * 
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.
 */
public class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if (root == null) return false;
        
        sum -= root.val;
        if (sum == 0 && isLeaf(root)) {
            return true;
        }
        boolean left = hasPathSum(root.left, sum);
        boolean right = hasPathSum(root.right, sum);
        
        return left || right;
    }
    
    public boolean isLeaf(TreeNode root){
        return (root.left == null) && (root.right == null);
    }
}