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Posts tagged as “array”

花花酱 LeetCode 1184. Distance Between Bus Stops

A bus has n stops numbered from 0 to n - 1 that form a circle. We know the distance between all pairs of neighboring stops where distance[i] is the distance between the stops number i and (i + 1) % n.

The bus goes along both directions i.e. clockwise and counterclockwise.

Return the shortest distance between the given start and destination stops.

Example 1:

Input: distance = [1,2,3,4], start = 0, destination = 1
Output: 1
Explanation: Distance between 0 and 1 is 1 or 9, minimum is 1.

Example 2:

Input: distance = [1,2,3,4], start = 0, destination = 2
Output: 3
Explanation: Distance between 0 and 2 is 3 or 7, minimum is 3.

Example 3:

Input: distance = [1,2,3,4], start = 0, destination = 3
Output: 4
Explanation: Distance between 0 and 3 is 6 or 4, minimum is 4.

Constraints:

  • 1 <= n <= 10^4
  • distance.length == n
  • 0 <= start, destination < n
  • 0 <= distance[i] <= 10^4

Solution: Summation

  1. compute the total sum
  2. compute the sum from s to d, c
  3. ans = min(c, sum – c)

Time complexity: O(d-s)
Space complexity: O(1)

C++

花花酱 LeetCode 34. Find First and Last Position of Element in Sorted Array

Given an array of integers nums sorted in ascending order, find the starting and ending position of a given target value.

Your algorithm’s runtime complexity must be in the order of O(log n).

If the target is not found in the array, return [-1, -1].

Example 1:

Input: nums = [5,7,7,8,8,10], target = 8
Output: [3,4]

Example 2:

Input: nums = [5,7,7,8,8,10], target = 6
Output: [-1,-1]

Solution: Binary Search

Basically this problem asks you to implement lower_bound and upper_bound using binary search.

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

C++

花花酱 LeetCode 217. Contains Duplicate

Given an array of integers, find if the array contains any duplicates.

Your function should return true if any value appears at least twice in the array, and it should return false if every element is distinct.

Example 1:

Input: [1,2,3,1]
Output: true

Example 2:

Input: [1,2,3,4]
Output: false

Example 3:

Input: [1,1,1,3,3,4,3,2,4,2]
Output: true

Solution 1: HashTable

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

C++

Solution 2: Sorting

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

C++

花花酱 LeetCode 1105. Filling Bookcase Shelves

We have a sequence of books: the i-th book has thickness books[i][0] and height books[i][1].

We want to place these books in order onto bookcase shelves that have total width shelf_width.

We choose some of the books to place on this shelf (such that the sum of their thickness is <= shelf_width), then build another level of shelf of the bookcase so that the total height of the bookcase has increased by the maximum height of the books we just put down.  We repeat this process until there are no more books to place.

Note again that at each step of the above process, the order of the books we place is the same order as the given sequence of books.  For example, if we have an ordered list of 5 books, we might place the first and second book onto the first shelf, the third book on the second shelf, and the fourth and fifth book on the last shelf.

Return the minimum possible height that the total bookshelf can be after placing shelves in this manner.

Example 1:

Input: books = [[1,1],[2,3],[2,3],[1,1],[1,1],[1,1],[1,2]], shelf_width = 4
Output: 6
Explanation:
The sum of the heights of the 3 shelves are 1 + 3 + 2 = 6.
Notice that book number 2 does not have to be on the first shelf.

Constraints:

  • 1 <= books.length <= 1000
  • 1 <= books[i][0] <= shelf_width <= 1000
  • 1 <= books[i][1] <= 1000

Solution: DP

dp[i] := min height of placing books[0] ~ books[i]
dp[-1] = 0
dp[j] = min{dp[i-1] + max(h[i] ~ h[j])}, 0 < i <= j, sum(w[i] ~ w[j]) <= shelf_width

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

C++

花花酱 1054. Distant Barcodes

In a warehouse, there is a row of barcodes, where the i-th barcode is barcodes[i].

Rearrange the barcodes so that no two adjacent barcodes are equal.  You may return any answer, and it is guaranteed an answer exists.

Example 1:

Input: [1,1,1,2,2,2]
Output: [2,1,2,1,2,1]

Example 2:

Input: [1,1,1,1,2,2,3,3]
Output: [1,3,1,3,2,1,2,1]

Note:

  1. 1 <= barcodes.length <= 10000
  2. 1 <= barcodes[i] <= 10000

Soluton: Sorting

Sort the element by their frequency in descending order. Fill the most frequent element first in even positions, if reach the end of the array, start from position 1 then 3, 5, …

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

C++

Solution 2: Find the most frequent

Actually, we only need to find the most frequent element and put in the even positions, then put the rest of the groups of elements in any order.

e.g. [1, 1, 2, 2, 2, 2, 2, 3, 4]
Can be
5*2 [2 – 2 – 2 – 2 – 2]
4*1 [2 4 2 – 2 – 2 – 2]
3*1 [2 4 2 3 2 – 2 – 2]
1*2 [ 2 3 2 3 2 1 2 1 2]

if we start with any other groups rather than 2, if will become:
[3 2 2 – 2 – 2 – 2 ] which is wrong…

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

C++