LeetCode in Kotlin

222. Count Complete Tree Nodes

Medium

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.

Solution

import com_github_leetcode.TreeNode

/*
 * Example:
 * var ti = TreeNode(5)
 * var v = ti.`val`
 * Definition for a binary tree node.
 * class TreeNode(var `val`: Int) {
 *     var left: TreeNode? = null
 *     var right: TreeNode? = null
 * }
 */
class Solution {
    fun countNodes(root: TreeNode?): Int {
        if (root == null) {
            return 0
        }
        val leftHeight = leftHeight(root)
        val rightHeight = rightHeight(root)
        // case 1: When Height(Left sub-tree) = Height(right sub-tree) 2^h - 1
        return if (leftHeight == rightHeight) {
            (1 shl leftHeight) - 1
        } else {
            1 + countNodes(root.left) + countNodes(root.right)
        }
    }

    private fun leftHeight(root: TreeNode?): Int {
        return if (root == null) {
            0
        } else {
            1 + leftHeight(root.left)
        }
    }

    private fun rightHeight(root: TreeNode?): Int {
        return if (root == null) {
            0
        } else {
            1 + rightHeight(root.right)
        }
    }
}

This site is open source. Improve this page.