HackerHeap

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.

Note: A leaf is a node with no children.

Example:

Input: [1,2,3]
    1
   / \
  2   3
Output: 25
Explanation:
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Therefore, sum = 12 + 13 = 25.

Example 2:

Input: [4,9,0,5,1]
    4
   / \
  9   0
 / \
5   1
Output: 1026
Explanation:
The root-to-leaf path 4->9->5 represents the number 495.
The root-to-leaf path 4->9->1 represents the number 491.
The root-to-leaf path 4->0 represents the number 40.
Therefore, sum = 495 + 491 + 40 = 1026.

Solution Using DFS:
We will DFS(Depth First Search) to find the numbers and add them to the sum, the time complexity would be O(N).

class HackerHeap {
    public int sumNumbers(TreeNode root) {
        if (root == null) return 0;
        return dfs(root, 0, 0);
    }
    public int dfs(TreeNode node, int num, int sum) {
        if(node == null) return sum;
        num = num*10 + node.val;
        if(node.left == null && node.right == null) {
            return sum+=num;
        }
        return dfs(node.left, num, sum) + dfs(node.right, num, sum); 
    }
}

In this video explanation we will solve Leetcode Sum Root to Leaf Numbers Java Solution