Given a binary tree containing digits from0-9only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path1->2->3which represents the number123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   / \
  2   3

The root-to-leaf path1->2represents the number12.
The root-to-leaf path1->3represents the number13.

Return the sum = 12 + 13 =25.

tag: binary tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {

    int ans = 0;
    public int sumNumbers(TreeNode root) {
        if (root == null) return 0;

        dfs(root, 0);

        return ans;
    }

    private void dfs(TreeNode root, int sum){
        if (root == null) return;

        sum = sum * 10 + root.val;

        if (root.left == null && root.right == null){
            ans += sum;
        }

        dfs(root.left, sum);
        dfs(root.right, sum);

        sum = (sum - root.val) / 10;
    }
}