回溯

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