Count Complete Tree Nodes LeetCode Solution | Easy Approach

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Count Complete Tree Nodes Given the root of a complete binary tree, return the number of the nodes in the tree.

According to Wikipedia, every level, except possibly the last, is completely filled in a complete binary tree, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h.

Design an algorithm that runs in less than O(n) time complexity.

Example 1:

Input: root = [1,2,3,4,5,6]
Output: 6

Example 2:

Input: root = []
Output: 0

Example 3:

Input: root = [1]
Output: 1

Constraints:

  • The number of nodes in the tree is in the range [0, 5 * 104].
  • 0 <= Node.val <= 5 * 104
  • The tree is guaranteed to be complete.

Count Complete Tree Nodes Solutions

Time: O(n)
Space: O(h)

C++

 class Solution {
 public:
  int countNodes(TreeNode* root) {
    if (!root)
      return 0;
    return 1 + countNodes(root->left) + countNodes(root->right);
  }
};

Java

 
class Solution {
  public int countNodes(TreeNode root) {
    if (root == null)
      return 0;
    return 1 + countNodes(root.left) + countNodes(root.right);
  }
}

Python


class Solution:
  def countNodes(self, root: Optional[TreeNode]) -> int:
    if not root:
      return 0
    return 1 + self.countNodes(root.left) + self.countNodes(root.right)

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