Sum Root to Leaf Numbers leetcode java

题目：

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.

题解：

分析这道题，由根节点往叶节点方向走，就是从高位往地位加和的方向。也就是说，当遍历的节点往叶节点方向走一层的时候，该节点的值应为父节点的值*10+当前节点的值。

由此可以写出代码：

 1     int sumhelper(TreeNode root, int levelBase) {
 2         if(root == null)
 3             return 0;
 4             
 5         if(root.left == null && root.right == null) {
 6             return levelBase + root.val; 
 7         }
 8         
 9         int nextLevelBase = (levelBase + root.val)*10 ;
         int leftSubTreeSum = sumhelper(root.left, nextLevelBase);
         int rightSubTreeSum = sumhelper(root.right, nextLevelBase);
     
         return leftSubTreeSum + rightSubTreeSum;
     }
     
     public int sumNumbers(TreeNode root) {
         return sumhelper(root,0);
     }