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 a

 class TreeNode {
     int val;
     TreeNode left;
     TreeNode right;
     TreeNode(int x) { val = x; }
 }
public class Solution {
    public int sumNumbers(TreeNode root) {
        if(root==null)return 0;
        return sumRoot(root,0);
    }

    private int sumRoot(TreeNode root, int sum) {
        if(root==null)return 0;
        sum=sum*10+root.val;//关键
        if(root.left==null&&root.right==null)return sum;
        return sumRoot(root.left, sum)+sumRoot(root.right, sum);//左右都要递归哦
    }

}