222. 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.

 

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.

 

Follow up: Traversing the tree to count the number of nodes in the tree is an easy solution but with O(n) complexity. Could you find a faster algorithm?

Rust Solution

struct Solution;
use rustgym_util::*;

trait Count {
    fn count(&self) -> i32;
}

impl Count for TreeLink {
    fn count(&self) -> i32 {
        if let Some(node) = self {
            1 + node.borrow().left.count() + node.borrow().right.count()
        } else {
            0
        }
    }
}

impl Solution {
    fn count_nodes(root: TreeLink) -> i32 {
        root.count()
    }
}

#[test]
fn test() {
    let root = tree!(1, tree!(2, tree!(4), tree!(5)), tree!(3, tree!(6), None));
    let res = 6;
    assert_eq!(Solution::count_nodes(root), res);
}