求根节点到叶节点数字之和

class Solution {
public:
    int result=0;
    void back(TreeNode*root, int num){
        if(root->left==NULL&&root->right==NULL){
            result+=(num*10+root->val);
            return;
        }
        if(root->left) back(root->left, num*10+root->val);
        if(root->right) back(root->right, num*10+root->val);
    }
    int sumNumbers(TreeNode* root) {
        back(root, 0);
        return result;
    }
};