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.

 class Solution {

     public boolean hasPathSum(TreeNode root, int sum) {

         if(root==null) return false;

         if (root.left==null && root.right==null && sum-root.val==) return true;

         return hasPathSum(root.left, sum-root.val) ||hasPathSum(root.right,sum-root.val);

     }

 }