Given a binary tree containing digits from0-9 only, each root-to-leaf path

could represent a number.

An example is the root-to-leaf path1->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 number12.

The root-to-leaf path 1->3 represents the number13.

Return the sum = 12 + 13 = 25.

解题思路:

    遍历二叉树，即可得知根到叶子节点的数，由于数据比较弱，路径上的数字用

int就能存下，如果树的深度大于10，int就会溢出，所以较好的做法是用字符串，

然后模拟加法运算即可.

解题代码:

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void dfs(TreeNode *rt,int path,int &sum)
    {
        if (!rt) return ;
        path = path * 10 + rt->val ;
        if (rt->left == rt->right && !rt->left)
            sum +=path;
        dfs(rt->left,path,sum);
        dfs(rt->right,path,sum);
    }
    int sumNumbers(TreeNode *root) 
    {
        int ans = 0 ;
        dfs(root,0,ans);
        return ans ;
    }
};