[leetcode] Sum Root to Leaf Numbers

Sum Root to Leaf Numbers

 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：

又是dfs。有一个地方开始错了，只有当遍历到叶子节点时，在对最后的结果进行处理，所以dfs函数中需要一个变量保存当前层某个节点时的值（这里是num）。

**
 * 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 *root, int num, int& res) {
        if(root->left==NULL && root->right==NULL)
            res = res + num*10 + root->val;
        if(root->left) 
            dfs(root->left, num*10+root->val, res);
        if(root->right)
            dfs(root->right, num*10+root->val, res);
    }
    int sumNumbers(TreeNode *root) {
        int res = 0;
        if(root==NULL)
            return res;
        dfs(root, 0, res);
        return res;
    }
};