Sum Root to Leaf Numbers

Q：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

A:回溯法

int helper(TreeNode *root, int num) {
    if (!root) return 0;
    num = num * 10 + root->val;
    if (root->left == nullptr && root->right == nullptr)
        return num;
    else
        return helper(root->left, num)
             + helper(root->right, num);
}

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