Repetitions Solution

3 September 2021


( The original statement can be found here )
You are given a DNA sequence: a string consisting of characters A, C, G, and T. Your task is to find the longest repetition in the sequence. This is a maximum-length substring containing only one type of character.

The only input line contains a string of n characters.

Print one integer: the length of the longest repetition.

1 ≤ n ≤ \(10^6\)
Time limit : 1.00s
Memory limit : 512MB


Input :

Output :


We are given a DNA sequence and asked to find the longest contiguous segment containing the same character.

Since \(N\) is at most \(10^6\), there is a simple linear approach to this problem.

One possible solution is to iterate through the string and have a variable that stores the character of the current substring. If the character at the \( ith\) iteration changes we update this variable, otherwise we increment the current count. At each iteration, we also update the answer to be \(max( answer, count)\).

The following animation illustrates this algorithm using the sample input.

#include <bits/stdc++.h>
using namespace std;

int main() {
    string s;
    char current;
    int count = 0, answer = 0;
    cin >> s;
    for (int i = 0; i < s.length(); i++) {
        if (s[i] != current) {
            current = s[i];
            count = 0;
        if (s[i] == current) {
        answer = max(answer, count);
    cout << answer;

This solution has both time and space complexity of \(O(N)\) where \(N\) is the length of the string.