Sum Root to Leaf Numbers leetcode

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.

用递归的方法来做, 把根节点到叶子节点所有值加起来, 递归条件是把当前的sum*10 加上子节点的值进行下一轮递归

结束条件是到子节点是空的时候就返回0
算法的本质是一次先序遍历，所以时间是O(n)，空间是栈大小，O(logn)。

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