# Posts tagged as “array”

There are n soldiers standing in a line. Each soldier is assigned a unique rating value.

You have to form a team of 3 soldiers amongst them under the following rules:

• Choose 3 soldiers with index (ijk) with rating (rating[i]rating[j]rating[k]).
• A team is valid if:  (rating[i] < rating[j] < rating[k]) or (rating[i] > rating[j] > rating[k]) where (0 <= i < j < k < n).

Return the number of teams you can form given the conditions. (soldiers can be part of multiple teams).

Example 1:

Input: rating = [2,5,3,4,1]
Output: 3
Explanation: We can form three teams given the conditions. (2,3,4), (5,4,1), (5,3,1).


Example 2:

Input: rating = [2,1,3]
Output: 0
Explanation: We can't form any team given the conditions.


Example 3:

Input: rating = [1,2,3,4]
Output: 4


Constraints:

• n == rating.length
• 1 <= n <= 200
• 1 <= rating[i] <= 10^5

## Solution 1: Brute Force

Time complexity: O(n^3)
Space complexity: O(1)

## Solution 2: Math

For each soldier j, count how many soldiers on his left has smaller ratings as left[j], count how many soldiers his right side has larger ratings as right[j]

ans = sum(left[j] * right[j] + (j – left[j]) * (n – j – 1 * right[j])

Time complexity: O(n^2)
Space complexity: O(1)

## C++

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 the array nums, for each nums[i] find out how many numbers in the array are smaller than it. That is, for each nums[i] you have to count the number of valid j's such that j != i and nums[j] < nums[i].

Return the answer in an array.

Example 1:

Input: nums = [8,1,2,2,3]
Output: [4,0,1,1,3]
Explanation:
For nums[0]=8 there exist four smaller numbers than it (1, 2, 2 and 3).
For nums[1]=1 does not exist any smaller number than it.
For nums[2]=2 there exist one smaller number than it (1).
For nums[3]=2 there exist one smaller number than it (1).
For nums[4]=3 there exist three smaller numbers than it (1, 2 and 2).


Example 2:

Input: nums = [6,5,4,8]
Output: [2,1,0,3]


Example 3:

Input: nums = [7,7,7,7]
Output: [0,0,0,0]


Constraints:

• 2 <= nums.length <= 500
• 0 <= nums[i] <= 100

## Solution 1: Brute Force

Time complexity: O(n^2)
Space complexity: O(1)

## Solution 2: Sort + Binary Search

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

## Solution 3: Cumulative frequency

Time complexity: O(n)
Space complexity: O(101)

## C++

Given an integer array arr. You have to sort the integers in the array in ascending order by the number of 1’s in their binary representation and in case of two or more integers have the same number of 1’s you have to sort them in ascending order.

Return the sorted array.

Example 1:

Input: arr = [0,1,2,3,4,5,6,7,8]
Output: [0,1,2,4,8,3,5,6,7]
Explantion: [0] is the only integer with 0 bits.
[1,2,4,8] all have 1 bit.
[3,5,6] have 2 bits.
[7] has 3 bits.
The sorted array by bits is [0,1,2,4,8,3,5,6,7]


Example 2:

Input: arr = [1024,512,256,128,64,32,16,8,4,2,1]
Output: [1,2,4,8,16,32,64,128,256,512,1024]
Explantion: All integers have 1 bit in the binary representation, you should just sort them in ascending order.


Example 3:

Input: arr = [10000,10000]
Output: [10000,10000]


Example 4:

Input: arr = [2,3,5,7,11,13,17,19]
Output: [2,3,5,17,7,11,13,19]


Example 5:

Input: arr = [10,100,1000,10000]
Output: [10,100,10000,1000]


Constraints:

• 1 <= arr.length <= 500
• 0 <= arr[i] <= 10^4

## Solution: Sorting

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

## Python3

Given a m * n matrix grid which is sorted in non-increasing order both row-wise and column-wise.

Return the number of negative numbers in grid.

Example 1:

Input: grid = [[4,3,2,-1],[3,2,1,-1],[1,1,-1,-2],[-1,-1,-2,-3]]
Output: 8
Explanation: There are 8 negatives number in the matrix.


Example 2:

Input: grid = [[3,2],[1,0]]
Output: 0


Example 3:

Input: grid = [[1,-1],[-1,-1]]
Output: 3


Example 4:

Input: grid = [[-1]]
Output: 1


Constraints:

• m == grid.length
• n == grid[i].length
• 1 <= m, n <= 100
• -100 <= grid[i][j] <= 100

## Solution 1: Brute Force

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

## Solution 2: Find the frontier

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

## C++

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