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 andsum = 22,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \      \
        7    2      1

return true, as there exist a root-to-leaf path5->4->11->2which sum is 22.

tag: binary tree

/**
 * 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;

        return dfs(root, root.val, sum);
    }

    private boolean dfs(TreeNode root, int path, int sum){
        if (root == null) return false;

        if (root.left == null && root.right == null){
            if (sum == path) return true;
            else return false;
        }

        boolean leftValid = root.left != null && dfs(root.left, path + root.left.val, sum);
        boolean rightValid = root.right != null && dfs(root.right, path + root.right.val, sum);

        return leftValid || rightValid;
    }
}