LeetCode 129. 求根节点到叶节点数字之和

LeetCode 129. 求根节点到叶节点数字之和
思路：
dfs,用sum去维护每棵树的和。

代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int res = 0;

    int sumNumbers(TreeNode* root) {
        if(!root) return 0;
        dfs(root,0);
        return res;
    }

    void dfs(TreeNode* root,int sum){
        sum = sum * 10 + root->val;
        //只有是根节点才不需要乘以倍数 否则都需要乘倍数
        if(!root->left && !root->right) res+=sum;
        if(root->left) dfs(root->left,sum);
        if(root->right) dfs(root->right,sum);
    }
};