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.

java代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private boolean stop = false;

    public boolean hasPathSum(TreeNode root, int sum) {
        calculate(root, 0, sum);
        return stop;
    }

    private void calculate(TreeNode node, int cur, int sum) {
        if (!stop && node != null) {
            if (node.left == null && node.right == null && (node.val + cur == sum)) {
                stop = true;
            }
            if (node.left != null) {
                calculate(node.left, cur+node.val, sum);
            }
            if (node.right != null) {
                calculate(node.right, cur+node.val, sum);
            }

        }

    }
}