# Posts tagged as “O(n)”

There is a room with n bulbs, numbered from 1 to n, arranged in a row from left to right. Initially, all the bulbs are turned off.

At moment k (for k from 0 to n - 1), we turn on the light[k] bulb. A bulb change color to blue only if it is on and all the previous bulbs (to the left) are turned on too.

Return the number of moments in which all turned on bulbs are blue.

Example 1:

Input: light = [2,1,3,5,4]
Output: 3
Explanation: All bulbs turned on, are blue at the moment 1, 2 and 4.


Example 2:

Input: light = [3,2,4,1,5]
Output: 2
Explanation: All bulbs turned on, are blue at the moment 3, and 4 (index-0).


Example 3:

Input: light = [4,1,2,3]
Output: 1
Explanation: All bulbs turned on, are blue at the moment 3 (index-0).
Bulb 4th changes to blue at the moment 3.


Example 4:

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


Example 5:

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


Constraints:

• n == light.length
• 1 <= n <= 5 * 10^4
• light is a permutation of  [1, 2, ..., n]

## Solution

Track the right most light l_k, all turned-on lights are blue if and only if the right most one is k, and there are exact k lights on right now.

Time complexity: O(n)
Space complexity: O(1)

## C++

Given an integer nreturn a string with n characters such that each character in such string occurs an odd number of times.

The returned string must contain only lowercase English letters. If there are multiples valid strings, return any of them.

Example 1:

Input: n = 4
Output: "pppz"
Explanation: "pppz" is a valid string since the character 'p' occurs three times and the character 'z' occurs once. Note that there are many other valid strings such as "ohhh" and "love".


Example 2:

Input: n = 2
Output: "xy"
Explanation: "xy" is a valid string since the characters 'x' and 'y' occur once. Note that there are many other valid strings such as "ag" and "ur".


Example 3:

Input: n = 7
Output: "holasss"


Constraints:

• 1 <= n <= 500

## Solution: Greedy

if n is odd, return n ‘a’s.
otherwise, return n -1 ‘a’s and 1 ‘b’

Time complexity: O(n)
Space complexity: O(n) or O(1)

## Python3

Given a binary tree root, a ZigZag path for a binary tree is defined as follow:

• Choose any node in the binary tree and a direction (right or left).
• If the current direction is right then move to the right child of the current node otherwise move to the left child.
• Change the direction from right to left or right to left.
• Repeat the second and third step until you can’t move in the tree.

Zigzag length is defined as the number of nodes visited – 1. (A single node has a length of 0).

Return the longest ZigZag path contained in that tree.

Example 1:

Input: root = [1,null,1,1,1,null,null,1,1,null,1,null,null,null,1,null,1]
Output: 3
Explanation: Longest ZigZag path in blue nodes (right -> left -> right).


Example 2:

Input: root = [1,1,1,null,1,null,null,1,1,null,1]
Output: 4
Explanation: Longest ZigZag path in blue nodes (left -> right -> left -> right).


Example 3:

Input: root = 
Output: 0


Constraints:

• Each tree has at most 50000 nodes..
• Each node’s value is between [1, 100].

## Solution: Recursion

For each node return
1. max ZigZag length if go left
2. max ZigZag length if go right
3. maz ZigZag length within the subtree

ZigZag(root):
ll, lr, lm = ZigZag(root.left)
rl, rr, rm = ZigZag(root.right)
return (lr+1, rl + 1, max(lr+1, rl+1, lm, rm))

Time complexity: O(n)
Space complexity: O(h)

## Python3

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted array: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

0
/ \
-3   9
/   /
-10  5

## Solution: Recursion

Recursively build a BST for a given range.

def build(nums, l, r):
if l > r: return None
m = l + (r – l) / 2
root = TreeNode(nums[m])
root.left = build(nums, l, m – 1)
root.right = build(nums, m + 1, r)
return root

return build(nums, 0, len(nums) – 1)

Time complexity: O(n)
Space complexity: O(logn)

## JavaScript

Given an array of integers nums, sort the array in ascending order.

Example 1:

Input: nums = [5,2,3,1]
Output: [1,2,3,5]


Example 2:

Input: nums = [5,1,1,2,0,0]
Output: [0,0,1,1,2,5]


Constraints:

• 1 <= nums.length <= 50000
• -50000 <= nums[i] <= 50000

Since n <= 50000, any O(n^2) won’t pass, we need O(nlogn) or better

## Solution 1: QuickSort

Time complexity: O(nlogn) ~ O(n^2)
Space complexity: O(logn) ~ O(n)

## Solution 2: Counting Sort

Time complexity: O(n)
Space complexity: O(max(nums) – min(nums))

## Solution 2: HeapSort

Time complexity: O(nlogn)
Space complexity: O(n)

## Solution 3: MergeSort

Time complexity: O(nlogn)
Space complexity: O(logn + n)

## Solution 4: BST

Time complexity: (nlogn)
Space complexity: O(n)