[LeetCode] Sum Root to Leaf Numbers

Problem:

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.

Solution:O(n)

public class Solution {
    public int sumNumbers(TreeNode root) {
        if(root == null)
            return 0;
        if(root.left == null && root.right == null)
            return root.val;
            
        int sum = 0;    
        if(root.left!=null){
            root.left.val += (root.val *10);
            sum+=sumNumbers(root.left);
        }
        if(root.right!=null){
            root.right.val += (root.val *10);
            sum+=sumNumbers(root.right);
        }
        return sum;
    }
}
Key Concept: Recursive and save prevalue to root.