题目描述：
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.
思路：
DFS问题，即从根节点出发进入统计（存为string s），如果左节点不为空，进入左子树统计；如果右子树不为空进入右子树统计。如果是叶子节点，记录状态放入总结果（List<string> result）。
遍历完之后，再遍历一遍result转为整形累加计算结果即可。
实现代码：/**
* Definition for a binary tree node.
* public class TreeNode {
* public int val;
* public TreeNode left;
* public TreeNode right;
* public TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int SumNumbers(TreeNode root)
{
if(root == null){
return 0;
}
var ret = new List<string>();
Sum(root, "",ref ret);
var s = 0;
for(var i = 0;i < ret.Count; i++){
s += int.Parse(ret[i]);
}
return s;
}
private void Sum(TreeNode root, string s,ref List<string> result)
{
if(root.left == null && root.right == null){
result.Add(s + root.val);
return;
}
if(root.left != null){
Sum(root.left, s + root.val, ref result);
}
if(root.right != null){
Sum(root.right, s + root.val, ref result);
}
}
}