Leetcode 222 - Count complete tree nodes

Note

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

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Input: root = [1,2,3,4,5,6]
Output: 6

Iteration

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/**
* Definition for a binary tree node.
* function TreeNode(val, left, right) {
* this.val = (val===undefined ? 0 : val)
* this.left = (left===undefined ? null : left)
* this.right = (right===undefined ? null : right)
* }
*/
/**
* @param {TreeNode} root
* @return {number}
*/
var countNodes = function(root) {
if (!root) return 0;
let result = 0;
let queue = [root];
while (queue.length) {
let length = queue.length;
result += length;
while(length--) {
let node = queue.shift();
node.left && queue.push(node.left);
node.right && queue.push(node.right);
}
}
return result;
};

Recursion

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/**
* @param {TreeNode} root
* @return {number}
*/
var countNodes = function(root) {
return count(root);

function count(node) {
if (!node) return 0;
return 1 + count(node.left) + count(node.right);
}
};