Sum Root to Leaf Numbers 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

.

Check the coding below in detail:

/*
Key to solve: Recursion + DFS
1.construct num=num*10+value every Tree level down
2.return sum when reach the bottom of tree
3.recursively through the TreeNode by DFS Traversal

*/
public class Solution {
public int sumNumbers(TreeNode root) {

return helperDFS(root,0,0);
}
private int helperDFS(TreeNode root, int num , int sum){
//base case
if(root==null) return 0;
num=num*10+root.val;
//check for reach the bottom
if(root.left==null && root.right==null){
sum+=num;
return sum;
}
return helperDFS(root.left,num,sum)+helperDFS(root.right,num,sum);
}
}