[LeetCode] Sum Root to Leaf Numbers

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.

Solution

class Solution {
    public int sumNumbers(TreeNode root) {
        if (root == null) return 0;
        return sum(root, 0);
    }
    private int sum(TreeNode root, int cur) {
        if (root == null) return 0;
        if (root.left == null && root.right == null) return cur*10+root.val;
        return sum(root.left, cur*10+root.val)+sum(root.right, cur*10+root.val);
    }
}