Count distinct elements in an array (A problem on sorting technique)

Problem

Write a function

def solution(A)

that, given an array A consisting of N integers, returns the number of distinct values in array A.

For example, given array A consisting of six elements such that:

A[0] = 2 A[1] = 1 A[2] = 1

A[3] = 2 A[4] = 3 A[5] = 1

the function should return 3, because there are 3 distinct values appearing in array A, namely 1, 2 and 3.

Write an efficient algorithm for the following assumptions:

N is an integer within the range [0..100,000];

each element of array A is an integer within the range [−1,000,000..1,000,000].

Solution (1) using set()

def solution(A):

    s = set(A)
    return len(s)

Detected time complexity:

O(N*log(N)) or O(N)

Test Cases

Correctness tests

extreme_empty: empty sequence

extreme_single: sequence of one element

extreme_two_elems: sequence of three distinct elements

extreme_one_value: sequence of 10 equal elements

extreme_negative: sequence of negative elements, length=5

extreme_big_values: sequence with big values, length=5

Medium1: chaotic sequence of value sfrom [0..1K], length=100

Medium2: chaotic sequence of value sfrom [0..1K], length=200

Medium3: chaotic sequence of values from [0..10], length=200

Performance tests

chaotic sequence of values from [0..100K], length=10K

large_random1

chaotic sequence of values from [-1M..1M], length=100K

large_random2

another chaotic sequence of values from [-1M..1M], length=100K

Sol (2) using dict() and then keys().len(): Straightforward to implement
Sol (3) without using set() or dict()

In this slide we discuss “Sol (3) without using set() or dict()”:

In this solution, we would use the sort() method of the array object.

def solution(A):
    A.sort()

    rtn = 1

    if len(A) == 0:
        return 0
    else:
        for i in range(1, len(A)):
            if A[i] != A[i-1]:
                rtn += 1

        return rtn

Tags: Technology,Algorithms,Python