LeetCode-Sum Root to Leaf Numbers

题目：https://oj.leetcode.com/problems/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.

源码：Java版本
算法分析：时间复杂度O(n)，空间复杂度O(logn)

/**
 * Definition for binary tree
 * 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);
    }
    
    private int sumNumbers(TreeNode node,int sum) {
        if(node==null) {
            return 0;
        }
        if(node.left==null && node.right==null) {
            return 10*sum+node.val;
        }
        return sumNumbers(node.left,10*sum+node.val)+
            sumNumbers(node.right,10*sum+node.val);
    }
}