[Leetcode] Sum Root to Leaf Numbers

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number 123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   / \
  2   3

The root-to-leaf path 1->2 represents the number 12.

The root-to-leaf path 1->3 represents the number 13.

Return the sum = 12 + 13 = 25.

From <http://oj.leetcode.com/problems/sum-root-to-leaf-numbers/>

Analysis: Every time the node reach a leaf, add the sum to the total; Otherwise, continue accumulating the sum value.

/**
 * 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 helper(TreeNode *root, int num, int& sum) {
        if (root == NULL)
            return;
        else 
        {
            num = num * 10 + root->val;
            if (root->left == NULL && root->right == NULL)
                sum += num;
            helper(root->left, num, sum);
            helper(root->right, num, sum);
        }
        
    }

    int sumNumbers(TreeNode *root) {
        int sum = 0;
        helper(root, 0, sum);
        return sum;
    }
};