129. Sum Root to Leaf Numbers

1. Description

You are given the root of a binary tree containing digits from 0 to 9 only.
Each root-to-leaf path in the tree represents a number.

• For example, the root-to-leaf path 1 -> 2 -> 3 represents the number 123.

Return the total sum of all root-to-leaf numbers. Test cases are generated so that the answer will fit in a 32-bit integer.
A leaf node is a node with no children.

2. Example

Example 1

Input: root = [1,2,3]
Output: 25
Explanation:
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Therefore, sum = 12 + 13 = 25.

Example 2

Input: root = [4,9,0,5,1]
Output: 1026
Explanation:
The root-to-leaf path 4->9->5 represents the number 495.
The root-to-leaf path 4->9->1 represents the number 491.
The root-to-leaf path 4->0 represents the number 40.
Therefore, sum = 495 + 491 + 40 = 1026.

3. Constraints

• The number of nodes in the tree is in the range [1, 1000].
• 0 <= Node.val <= 9
• The depth of the tree will not exceed 10.

4. Solutions

Depth-first Search

n is the number of nodes in the tree
Time complexity: O(n)
Space complexity: O(logn->n)

class Solution {
public:
    int sumNumbers(TreeNode *root) {
        int path_sum = 0;
        return get_tree_sum(root, path_sum);
    }

private:
    int get_tree_sum(TreeNode *root, int path_sum) {
        if (root != nullptr) {
            path_sum = path_sum * 10 + root->val;

            if (root->left == nullptr && root->right == nullptr) {
                return path_sum;
            } else {
                return get_tree_sum(root->left, path_sum) + get_tree_sum(root->right, path_sum);
            }
        } else {
            return 0;
        }
    }
};