Sum of Root To Leaf Binary Numbers

LeetCode Q 1022 - Sum of Root To Leaf Binary Numbers

Given a binary tree, each node has value 0 or 1. Each root-to-leaf path represents a binary number starting with the most significant bit. For example, if the path is 0 -> 1 -> 1 -> 0 -> 1, then this could represent 01101 in binary, which is 13.
For all leaves in the tree, consider the numbers represented by the path from the root to that leaf.
Return the sum of these numbers.

Example 1: Input: [1,0,1,0,1,0,1] ; Output: 22
Explanation: (100) + (101) + (110) + (111) = 4 + 5 + 6 + 7 = 22

Note:

• The number of nodes in the tree is between 1 and 1000.
• node.val is 0 or 1.
• The answer will not exceed 2^31 - 1.

Solution: DFS

Code:

private int sum = 0;
public int sumRootToLeaf(TreeNode root) {
  dfs(root, 0);
  return sum;
}

private void dfs (TreeNode root, int number) {
  
  number = number * 2 + root.val;

  if (root.left == null && root.right == null) {
    sum += number; return;
  }

  if (root.left != null) dfs(root.left, number); // avoid null pointer exception in the 1st line of this code block

  if (root.right != null) dfs(root.right, number);
}