Grokking Data Structures & Algorithms for Coding Interviews
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Advanced Sorting Techniques
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Now, let's explore advanced sorting techniques that go beyond simple comparison-based sorting methods like bubble, merge, and quick sort. Advanced sorting techniques often handle specific types of data or optimize sorting under unique constraints. They are typically non-comparison sorts that can achieve lower time complexities under certain conditions.

In this lesson, we will cover three significant advanced sorting algorithms:

  • Counting Sort: An integer sorting algorithm that operates with key assumption about the range of the input data.
  • Radix Sort: A non-comparative sorting algorithm that sorts integers digit by digit starting from the least significant digit to the most.
  • Bucket Sort: Also known as bin sort, it distributes elements into various 'buckets' which are then sorted using another sort, typically insertion sort.

Each of these sorting algorithms will be introduced with their context of use, general working mechanism, a detailed step-by-step guide, sample code implementation, and a complexity analysis.

Counting Sort

Counting sort calculates the number of occurrences of each distinct element in the array to sort the array. It then uses arithmetic to determine the positions of each element in the output sequence. This sort is efficient when the range of input data is not significantly greater than the number of objects to be sorted.

Step-by-Step Algorithm

  1. Determine Range:

    • Identify the Maximum Value: Find the largest value in the input array to determine the range of possible values.
    • Create Count Array: Establish an array, count, with a size of max + 1 (to account for zero indexing) initialized to zero.

  2. Count Occurrences:

    • Traverse Input Array: For each element x in the input array, increment count[x].
    • Purpose: This step tallies the number of times each value appears in the input array.

  3. Accumulate Counts:

    • Modify Count Array: Transform each index in the count array to be the sum of the previous indices.
    • Key Operation: count[i] += count[i - 1].
    • Outcome: After this step, count[i] tells the number of elements less than or equal to i.

  4. Build the Output Array:

    • Initialize Output Array: Create an array output that will store the sorted elements.
    • Place Elements Correctly: Iterate through the input array from the last element to the first (to maintain stability):
      • Place the element in the correct position in the output array: output[count[arr[i]] - 1] = arr[i].
      • Decrement the count in the count array: count[arr[i]]--.
      • Highlight: This placement ensures elements are sorted and that the sort is stable (maintains the relative order of duplicate values).

  5. Copy to Original Array:

    • Final Step: Copy the sorted elements from the output array back to the original input array.

Code

Python3
Python3
. . . .

Complexity Analysis

  • Time Complexity: O(n + k), where n is the number of elements and k is the range of the input.
  • Space Complexity: O(k), due to the additional space used by the count array.

Radix Sort

Radix sort is a non-comparative sorting algorithm that sorts numbers digit by digit starting from the least significant digit to the most significant digit. Radix sort uses counting sort as an intermediate sorting method to sort digits. Because it processes individual digits, it can handle large numbers efficiently, provided the digits are uniformly distributed.

Step-by-Step Algorithm

  1. Find the Maximum Number:

    • Determine the maximum number in the array to know the number of digits in the longest number.

  2. Sort by Each Digit:

    • For each digit position, starting from the least significant digit:
      • Apply a stable sort (counting sort) to sort all the numbers according to the digit at the current position.

  3. Counting Sort for Digits:

    • Instead of sorting by full value, the counting sort is applied to individual digits represented in each position.
    • Use a count array of size 10 (for decimal system) to keep track of the number of occurrences of each digit.

  4. Repeat for All Digit Positions:

    • Incrementally move to the next significant digit and repeat the sorting process.
    • Continue until the most significant digit has been sorted.

Code

Python3
Python3
. . . .

Complexity Analysis

  • Time Complexity: O(d*(n+b)), where d is the number of digits in the largest number, n is the number of elements in the array, and b is the base of the numbering system used. For the decimal system, b is 10.
  • Space Complexity: O(n + b), due to the temporary output array and the count arrays used for each digit.

Radix sort is particularly useful when sorting large numbers or strings of characters that can be treated as numbers. It avoids the comparisons typical of other sorting algorithms, making it uniquely fast for specific types of data. However, its efficiency depends heavily on the digit size (d) and the base (b), which determine its appropriateness for a given application.

Bucket Sort

Bucket sort, also known as bin sort, is an efficient sorting algorithm that distributes elements of an array into a number of buckets. Each bucket is then sorted individually, either using a different sorting algorithm or by recursively applying the bucket sort. This method is useful when the input is uniformly distributed over a range.

Step-by-Step Algorithm

  1. Create Buckets:

    • Determine the number of buckets, k, typically equal to the square root of the number of elements in the array.
    • Create an array of buckets where each bucket is a list that will hold elements of the array.

  2. Distribute Elements:

    • For each element in the array, find the appropriate bucket based on its value. This can be calculated using a function like index = floor(value * k / (maxValue + 1)).
    • Insert the element into its corresponding bucket.

  3. Sort Each Bucket:

    • Sort the elements in each bucket. This can be done using a simple sorting algorithm like insertion sort for efficiency because each bucket is expected to be small.

  4. Concatenate Buckets:

    • Once all buckets are sorted, concatenate them back into the original array in order. This effectively compiles the sorted elements into a single sorted array.

Code

Python3
Python3
. . . .

Complexity Analysis

  • Time Complexity: The average case is O(n + n^2/k + k), where k is the number of buckets. If k approaches n, the time complexity nears O(n).
  • Space Complexity: O(n + k), due to the space needed for the buckets and the elements they contain.

Bucket sort is especially effective when the input is uniformly distributed across a range. It excels at sorting floating-point numbers and can be significantly faster than comparative sorting algorithms like quicksort and mergesort in the right contexts. For example, it's commonly used in scenarios like graphics where precision and performance are crucial, and data sets are often distributed uniformly.

Now, let's start solving the problems of sorting techniques.

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