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.

给定一个仅包含0-9数字的二叉树，每个根到叶的路径可以代表一个数。

一个例子 根到叶的路径 1->2->3 代表数123.

找出所有根到叶 数的和。

想到就是深度优先遍历。

	public int sumNumbers(TreeNode root) {
		return dfs(root, 0);
	}

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