leetcode------Sum Root to Leaf Numbers

标题：	Sum Root to Leaf Numbers
通过率：	30.4%
难度：	中等

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.

整个过程是深度优先，然后每下一层就放大十倍，那么如果是叶子节点则与sum相加，其他时候继续往下搜索，代码如下：

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