Sum Root to Leaf Numbers

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.

Solution:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumNumbers(TreeNode root) {
        return sumNumbers(root, 0);
    }
    public int sumNumbers(TreeNode root, int sum) {
        if(root==null) return 0;
        if(root.left==null && root.right==null) return sum*10+root.val;
        else return sumNumbers(root.left, sum*10+root.val) + sumNumbers(root.right, sum*10+root.val);
    }
}
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