LeetCode-in-Go
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
/**
* Definition for a binary tree node.
* type TreeNode struct {
* Val int
* Left *TreeNode
* Right *TreeNode
* }
*/
func countNodes(root *TreeNode) int {
if root == nil {
return 0
}
leftHeight := leftHeight(root)
rightHeight := rightHeight(root)
// case 1: When Height(Left sub-tree) = Height(right sub-tree) 2^h - 1
if leftHeight == rightHeight {
return (1 << leftHeight) - 1
} else {
return 1 + countNodes(root.Left) + countNodes(root.Right)
}
}
func leftHeight(root *TreeNode) int {
if root == nil {
return 0
}
return 1 + leftHeight(root.Left)
}
func rightHeight(root *TreeNode) int {
if root == nil {
return 0
}
return 1 + rightHeight(root.Right)
}