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.

 

/**
 * 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 solve(root, 0);
    }

	private int solve(TreeNode root, int level) {
		if (root == null) {
        	return 0;
        }
        if (root.left==null && root.right==null) {
        	return  level + root.val;
        }
        int nextLevel = (level + root.val) * 10;
        int left = solve(root.left, nextLevel);
        int right = solve(root.right, nextLevel);
        return left + right;
	}
}