How do optimise this loop for O(n) or O(nlogn)

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Guys I wanna perform this loop hopefully in O(n). I just need to calculate the minimum of distance between the second and the first occurrence of all elements in an array.

for example abcdeabb the answer for m = 1

What I have tried:

C++

m = INT_MAX
for(int i = 0; i<s.length(); ++i)
{
    for(int j = i+1; j<s.length(); ++j)
    {
        if(s[i] == s[j] and m > j - i)
        {
            m = j - i;
            if(m == 1)
                break;
        }
    }
}

Posted 13-Dec-19 7:55am

Md Razeenuddin Mehdi

Updated 17-Dec-19 22:16pm

Rick York

v2

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Patrice T · Answer 1 · 2019-12-13T12:57:00

Quote:
How do optimise this loop for O(n) or O(nlogn)

Rule of seasoned programmers: first, make it right, then make it fast.

Quote:
for example abcdeabb the answer for m = 1

There is no way the answer is 1 !
The example have 2 matches

First match is abcdeabb
               ^    ^
and second match is abcdeabb
                     ^    ^

To get answer = 1

the match is abcdeabb
                   ^^

and this is second and third occurrence of b.
Your algorithm is correct if a letter appear no more than 2 times is string.

Instead of checking a letter with remainder of string, my first approach would be to check a letter with beginning of string to ensure that a letter is checked with first occurrence.

I don't think your goal performance is possible.

Stefan_Lang · Answer 2 · 2019-12-17T22:16:00

You can make it O(N+C) where C is the number of valid characters in your alphabet. Try this:

C++

#include <iostream>
#include <limits>

int min_repetitions(char const* text) {

    int min_rep = -1; // indicates that no repetitions were found

    if (!text)
        return -1;

    std::ptrdiff_t positions[256] = {0 }; // o(number of valid characters)

    min_rep = std::numeric_limits<int>::max();
    bool repetition_found = false;
    
    for (char const* ptext = text; *ptext != 0; ++ptext) {
        char c = *ptext;
        if (positions[c]==0) {
            positions[c]=ptext-text;
        }
        else if (positions[c]>0) {
            int distance = ptext-text - positions[c];
            if (distance < min_rep) {
                min_rep = distance;
                repetition_found = true;
            }
            positions[c] = -1; // disable future checks
        }
    }
    
    if (!repetition_found)
        min_rep = -1;

    return min_rep;
}
int main()
{
    char test[] = "abcdeabb";
 
    int min_rep = min_repetitions(test);
    if (min_rep >= 0)
        std::cout << "Minimum repetition distance for " << test << " is: " << min_rep << std::endl;
    else
        std::cout << "The text "<< test << " has no repetitions" << std:.endl;

    return 0;
}

How do optimise this loop for O(n) or O(nlogn)

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