题目：

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：

没什么好说的，还是递归。

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:
    int sumNumbers(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(root == NULL)
        {
            return 0;
        }
        return num(root, 0);
    }
    
    int num(TreeNode * p, int n)
    {
        if(p->left == NULL && p->right == NULL)
        {
            return 10 * n + p->val;
        }
        else if(p -> left == NULL)
        {
            return num(p->right, 10 * n + p->val);
        }
        else if(p -> right == NULL)
        {
            return num(p->left, 10 * n + p->val);
        }
        else
        {
            return num(p->left, 10 * n + p->val) + num(p->right, 10 * n + p->val);
        }
    }
};