Count complete tree nodes

Binary Search Bit Manipulation Tree Binary Tree

72.1% acceptance

Feb 27, 2026

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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.

Solution

Rust

Time O(n)

Space O(n)

LeetCode

solution.rs

// Definition for a binary tree node.
// #[derive(Debug, PartialEq, Eq)]
// pub struct TreeNode {
//   pub val: i32,
//   pub left: Option<Rc<RefCell<TreeNode>>>,
//   pub right: Option<Rc<RefCell<TreeNode>>>,
// }
// 
// impl TreeNode {
//   #[inline]
//   pub fn new(val: i32) -> Self {
//     TreeNode {
//       val,
//       left: None,
//       right: None
//     }
//   }
// }
use std::rc::Rc;
use std::cell::RefCell;
impl Solution {
  pub fn count_nodes(root: Option<Rc<RefCell<TreeNode>>>) -> i32 {
    match root {
      None => 0,
      Some(node) => {
        let node = node.borrow();
        1 + Self::count_nodes(node.left.clone()) + Self::count_nodes(node.right.clone())
      }
    }
  }
}