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.

思路： 每个节点表示的值为  上一个节点的基数 * 10 + 该节点的值，用一个变量sum（base）来记录上一个节点的基数

java代码：

1. int helper(TreeNode root,int sum) {
2. if(root==null) return sum;
3. if(root.left==null && root.right==null) {
4. return sum * 10 + root.val;
5. }
6. int left = 0;
7. int right = 0;
8. if(root.left!=null) {
9. left = helper(root.left,sum*10+root.val);
10. }
11. if(root.right!=null) {
12. right = helper(root.right,sum*10+root.val);
13. }
14. return left + right;
15. }
16. public int sumNumbers(TreeNode root) {
17. if(root==null) return 0;
18. if(root.left==null && root.right==null) return root.val;
19. return helper(root,0);
20. }