Leetcode 129 Sum Root to Leaf Numbers

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number 123.

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

For example,

    1
   / \
  2   3

The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.

Return the sum = 12 + 13 = 25.

Solution 1: DFS, when reach to leaf, add the result.

 public class Solution {  
   public int sumNumbers(TreeNode root) {  
     int[] res=new int[1];  
     dfs(root,0,res);  
     return res[0];  
   }  
   private void dfs(TreeNode x, int num, int[] res) {  
     if (x==null) return;  
     num=num*10+x.val;  
     if (x.left==null && x.right==null) res[0]+=num;  
     dfs(x.left,num,res);  
     dfs(x.right,num,res);  
   }  
 }