Program to find number of only child in a binary tree in python

Suppose we have a binary tree; we have to find the number of nodes that are an only child. As we know a node x is called an only child node when its parent has exactly one child that is x.

So, if the input is like

then the output will be 2 as 8 and 6 are the only children.

To solve this, we will follow these steps:

• if root is null, then
  • return 0
• d := a double ended queue
• insert root at the end of d
• count := 0
• while d is not empty, do
  • current := left element of d and delete left element
  • if left of current is not null, then
    • insert left of current into d
    • if right of current is null, then
      • count := count + 1
    • if right of current is not null, then
      • insert right of current into d
      • if left of current is null, then
        • count := count + 1
• return count

Let us see the following implementation to get better understanding:

Example Code

from collections import deque

class TreeNode:
   def __init__(self, data, left = None, right = None):
      self.data = data
      self.left = left
      self.right = right

class Solution:
   def solve(self, root):
      if not root:
         return 0
      d = deque()
      d.append(root)
      count = 0
      while d:
         current = d.popleft()
         if current.left:
            d.append(current.left)
            if not current.right:
               count += 1
            if current.right:
               d.append(current.right)
               if not current.left:
                  count += 1
         return count

ob = Solution()
root = TreeNode(9)
root.left = TreeNode(7)
root.right = TreeNode(10)
root.left.right = TreeNode(8)
root.right.right = TreeNode(6)
print(ob.solve(root))

Input

root = TreeNode(9)
root.left = TreeNode(7)
root.right = TreeNode(10)
root.left.right = TreeNode(8)
root.right.right = TreeNode(6)

Output

2