LeetCode 129 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

.

public int sumNumbers(TreeNode root) {
if (root == null) return 0;
if (root.left == null && root.right == null) return root.val;
if (root.left != null) root.left.val += root.val * 10;
if (root.right != null) root.right.val += root.val * 10;
return sumNumbers(root.left) + sumNumbers(root.right);
}