222. Count Complete Tree Nodes

Level : Medium

原題連結 : Click

題目 :

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 :

解題思路 :

• 這題Recursive解非常簡潔且易懂~
• 每經過一個node便+1

Recursive解法

• time complexity: O(n) , space complexity: O(1)

Runtime: 12 ms, faster than 99.33% of Go online submissions for Count Complete Tree Nodes.

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/**
 * 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
    }
    
    return 1 + countNodes(root.Left) + countNodes(root.Right)

}

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