Sum Root to Leaf Numbers

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該題可以用DFS解決，在DFS時記錄path，當到達leaf時將path所表示的數加到sum上。

 1 class Solution { 2 public: 3     int sumNumbers(TreeNode *root) { 4         vector<int> path; 5         int sum = 0; 6         dfs(root,path,sum); 7         return sum; 8     } 9     void dfs(TreeNode * root, vector<int> & path, int & sum){10         if(!root) return;11         path.push_back(root->val);12         if(!root->left && !root->right){13             int len = path.size();14             for(int i = 0; i < len; i++)15                 sum += path[i]*pow(10.0,len-i-1);16         }17         if(root->left) dfs(root->left,path,sum);18         if(root->right) dfs(root->right,path,sum);19         path.pop_back();20     }21 };