LeetCode -- Sum Root to Leaf Numbers

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);
}
}
}