【Leetcode】Sum Root to Leaf Numbers

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.

解题思路：用DFS算法搜索每条路经上的数，求出每条路经数字组合在一起的数，再将这些数相加。

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) {
        int Sum=0,CurrSum=0;
        build_path(Sum, CurrSum, root);
        return Sum;
    }
private:
    void build_path(int &Sum, int &CurrSum, TreeNode *root){
        if(root==nullptr)return;
        CurrSum=CurrSum*10+root->val;
        if(!root->left&&!root->right){
            Sum+=CurrSum;
        }else{
            build_path(Sum, CurrSum, root->left);
            build_path(Sum, CurrSum, root->right);
        }
        CurrSum=(CurrSum-root->val)/10;
    }
};