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);
}