【leetcode】Sum Root to Leaf Numbers

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/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void calSum(TreeNode *root, int val, int &sum){
        if(root == NULL){
            return;
        }
        
        if(root->left == NULL && root->right == NULL){
            sum += root->val + val * 10;
            return;
        }
        
        calSum(root->left, val * 10 + root->val, sum);
        calSum(root->right, val * 10 + root->val, sum);
    }
    
    int sumNumbers(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int sum = 0;
        
        calSum(root, 0, sum);
        
        return sum;
    }
};