題目：
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.
題解：
分析這道題，由根節點往葉節點方向走，就是從高位往地位加和的方向。也就是說，當遍歷的節點往葉節點方向走一層的時候，該節點的值應為父節點的值*10+當前節點的值。
由此可以寫出代碼：
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 ;
10 int leftSubTreeSum = sumhelper(root.left, nextLevelBase);
11 int rightSubTreeSum = sumhelper(root.right, nextLevelBase);
12
13 return leftSubTreeSum + rightSubTreeSum;
14 }
15
16 public int sumNumbers(TreeNode root) {
17 return sumhelper(root,0);
18 }
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 ;
10 int leftSubTreeSum = sumhelper(root.left, nextLevelBase);
11 int rightSubTreeSum = sumhelper(root.right, nextLevelBase);
12
13 return leftSubTreeSum + rightSubTreeSum;
14 }
15
16 public int sumNumbers(TreeNode root) {
17 return sumhelper(root,0);
18 }