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);}}}