129. Sum Root to Leaf Numbers Leetcode Java

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:

Recursion:

Recursion equation: V(root)=curSUM*10+V(left)+V(right). 

Base case: root==null-> return 0, if root is leaf, return curSUM*10+root.val;

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