树——sum-root-to-leaf-numbers（根到叶节点数字之和）

问题：

　　Given a binary tree containing digits from0-9only, each root-to-leaf path could represent a number.

　　An example is the root-to-leaf path1->2->3which represents the number123.

　　Find the total sum of all root-to-leaf numbers.

　　For example,

　　    1
　　   / 
　　  2   3

　　The root-to-leaf path1->2represents the number12.
　　The root-to-leaf path1->3represents the number13.

　　Return the sum = 12 + 13 =25.

代码：

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumNumbers(TreeNode root) {
       int sum = 0;
       if(root==null)
           return sum;
       return sumNumbers(root,sum);
    }
   
    public int sumNumbers(TreeNode root, int sum){
        if(root==null)
            return 0;
        sum = sum*10+root.val;
        if(root.left==null && root.right==null)
            return sum;
        return sumNumbers(root.left, sum) + sumNumbers(root.right, sum);
    }
}