129. Sum Root to Leaf Numbers

• Difficulty: Medium

• Topics: Tree, Depth-first Search

• Similar Questions:

  • Path Sum

  • Binary Tree Maximum Path Sum

  • Smallest String Starting From Leaf

Problem:

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.

Note: A leaf is a node with no children.

Example:

Input: [1,2,3]
    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: [4,9,0,5,1]
    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.

Solutions:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int sumNumbers(TreeNode* root) {
        int ret = 0;
        helper(root, 0, ret);
        return ret;
    }

    void helper(TreeNode* root, int path, int& ret) {
        if (root == nullptr) {
            return;
        }

        int val = 10 * path + root->val;

        if (root->left == nullptr && root->right == nullptr) {
            ret += val;
            return;
        }

        if (root->left) {
            helper(root->left, val, ret);
        }

        if (root->right) {
            helper(root->right, val, ret);
        }

    }
};