Substring of length K having maximum frequency in the given string
Last Updated :
15 Jul, 2025
Given a string str, the task is to find the substring of length K which occurs the maximum number of times. If more than one string occurs maximum number of times, then print the lexicographically smallest substring.
Examples:
Input: str = "bbbbbaaaaabbabababa", K = 5
Output: ababa
Explanation:
The substrings of length 5 from the above strings are {bbbbb, bbbba, bbbaa, bbaaa, baaaa, aaaaa, aaaab, aaabb, aabba, abbab, bbaba, babab, ababa, babab, ababa}.
Among all of them, substrings {ababa, babab} occurs the maximum number of times(= 2).
The lexicographically smallest string from {ababa, babab} is ababa.
Therefore, "ababa" is the required answer.
Input: str = "heisagoodboy", K = 5
Output: agood
Explanation:
The substrings of length 5 from the above string are {heisa, eisag, isago, sagoo, agood, goodb, oodbo, odboy}.
All of them occur only once. But the lexicographically smallest string among them is "agood".
Therefore, "agood" is the required answer.
Naive Approach: The simplest approach to solve the problem is to generate all the substrings of size K from the given string and store the frequency of each substring in a Map. Then, traverse the Map and find the lexicographically smallest substring which occurs maximum number of times and print it.
C++
// C++ implementation to find
// the maximum occurring character in
// an input string which is lexicographically first
#include <bits/stdc++.h>
using namespace std;
// function to find the maximum occurring character in
// an input string which is lexicographically first
string maximumOccurringString(string str, int k)
{
// store current string
string curr= "";
int i=0, j=0, n=str.length();
// to store all substring and there number of occurrences
// also use map because it stores all strings in lexographical order
map<string,int>mp;
// sliding window approach to generate all substring
while(j<n){
curr+=str[j];
// window size less then k so increase only 'j'
if(j-i+1 < k){
j++;
}
// window size is equal to k
// put current string into map and slide the window
// by incrementing 'i' and 'j' to generate all substring
else if(j-i+1 == k){
mp[curr]++;
curr.erase(0, 1);
i++;
j++;
}
}
// to count the maximum occurring string
int cnt=-1;
// to store the maximum occurring string
string ans;
for(auto x : mp){
int c = x.second;
if(c > cnt){
ans = x.first;
cnt =c;
}
}
// return the maximum occurring string
return ans;
}
// Driver Code
int main()
{
// Given string
string str = "bbbbbaaaaabbabababa";
// Given K size of substring
int k = 5;
// Function Call
cout << maximumOccurringString(str, k);
return 0;
}
//this code is contributed by bhardwajji
import java.util.*;
public class Main {
// function to find the maximum occurring character in
// an input string which is lexicographically first
static String maximumOccurringString(String str, int k) {
// store current string
String curr = "";
int i = 0, j = 0, n = str.length();
// to store all substring and there number of occurrences
// also use TreeMap because it stores all strings in lexicographical order
TreeMap<String, Integer> mp = new TreeMap<>();
// sliding window approach to generate all substring
while (j < n) {
curr += str.charAt(j);
// window size less then k so increase only 'j'
if (j - i + 1 < k) {
j++;
}
// window size is equal to k
// put current string into map and slide the window
// by incrementing 'i' and 'j' to generate all substring
else if (j - i + 1 == k) {
mp.put(curr, mp.getOrDefault(curr, 0) + 1);
curr = curr.substring(1);
i++;
j++;
}
}
// to count the maximum occurring string
int cnt = -1;
// to store the maximum occurring string
String ans = "";
for (Map.Entry<String, Integer> x : mp.entrySet()) {
int c = x.getValue();
if (c > cnt) {
ans = x.getKey();
cnt = c;
}
}
// return the maximum occurring string
return ans;
}
// Driver Code
public static void main(String[] args) {
// Given string
String str = "bbbbbaaaaabbabababa";
// Given K size of substring
int k = 5;
// Function Call
System.out.println(maximumOccurringString(str, k));
}
}
# Python3 implementation to find
#the maximum occurring character in
#an input string which is lexicographically first
# function to find the maximum occurring character in
# an input string which is lexicographically first
def maximum_occuring_string(string, k):
# store current string
curr = ""
n = len(string)
i = j = 0
# to store all substring and there number of occurrences
# also use map because it stores all strings in lexographical order
mp = {}
# sliding window approach to generate all substring
while j < n:
curr += string[j]
# window size less then k so increase only 'j'
if j - i + 1 < k:
j += 1
# window size is equal to k
# put current string into map and slide the window
# by incrementing 'i' and 'j' to generate all substring
elif j - i + 1 == k:
if curr in mp:
mp[curr] += 1
else:
mp[curr] = 1
curr = curr[1:]
i += 1
j += 1
#o count the maximum occurring string
cnt = -1
ans = ""
for x in mp:
c = mp[x]
if c > cnt or (c == cnt and x < ans):
ans = x
cnt = c
return ans
# Driver code
string = "bbbbbaaaaabbabababa"
k = 5
print(maximum_occuring_string(string, k))
using System;
using System.Collections.Generic;
class Program
{
// function to find the maximum occurring character in
// an input string which is lexicographically first
static string MaximumOccurringString(string str, int k)
{
// store current string
string curr = "";
int i = 0, j = 0, n = str.Length;
// to store all substring and there number of occurrences
// also use SortedDictionary because it stores all strings in lexographical order
SortedDictionary<string, int> mp = new SortedDictionary<string, int>();
// sliding window approach to generate all substring
while (j < n)
{
curr += str[j];
// window size less then k so increase only 'j'
if (j - i + 1 < k)
{
j++;
}
// window size is equal to k
// put current string into map and slide the window
// by incrementing 'i' and 'j' to generate all substring
else if (j - i + 1 == k)
{
if (mp.ContainsKey(curr))
{
mp[curr]++;
}
else
{
mp.Add(curr, 1);
}
curr = curr.Substring(1);
i++;
j++;
}
}
// to count the maximum occurring string
int cnt = -1;
// to store the maximum occurring string
string ans = "";
foreach (var x in mp)
{
int c = x.Value;
if (c > cnt)
{
ans = x.Key;
cnt = c;
}
}
// return the maximum occurring string
return ans;
}
// Driver Code
static void Main()
{
// Given string
string str = "bbbbbaaaaabbabababa";
// Given K size of substring
int k = 5;
// Function Call
Console.WriteLine(MaximumOccurringString(str, k));
}
}
// function to find the maximum occurring character in
// an input string which is lexicographically first
function MaximumOccurringString(str, k) {
// store current string
let curr = "";
let i = 0, j = 0, n = str.length;
// to store all substring and there number of occurrences
// also use Map because it stores all strings in lexographical order
let mp = new Map();
// sliding window approach to generate all substring
while (j < n) {
curr += str[j];
// window size less then k so increase only 'j'
if (j - i + 1 < k) {
j++;
}
// window size is equal to k
// put current string into map and slide the window
// by incrementing 'i' and 'j' to generate all substring
else if (j - i + 1 == k) {
if (mp.has(curr)) {
mp.set(curr, mp.get(curr) + 1);
}
else {
mp.set(curr, 1);
}
curr = curr.substring(1);
i++;
j++;
}
}
// to count the maximum occurring string
let cnt = -1;
// to store the maximum occurring string
let ans = "";
let keys = Array.from(mp.keys())
keys.sort()
//console.log(keys)
for (let key of keys) {
let c = mp.get(key);
if (c > cnt) {
ans = key;
cnt = c;
}
}
// return the maximum occurring string
return ans;
}
// Given string
let str = "bbbbbaaaaabbabababa";
// Given K size of substring
let k = 5;
// Function Call
console.log(MaximumOccurringString(str, k));
Time Complexity: O(N*( K + log K))
Auxiliary Space: O(N * K)
Efficient Approach: To optimize the above approach, the idea is to use Sliding Window technique. Consider a window of size
K to generate all substrings of length K and count the frequency of a substring generated in a Map. Traverse the map and find the substring that occurs maximum number of times and print it. If several of them exist, then print the lexicographically smallest substring.
Below is the implementation of the above approach.
C++
// C++ program for the above approach
#include <bits/stdc++.h>
using ll = long long int;
using namespace std;
// Function that generates substring
// of length K that occurs maximum times
void maximumOccurringString(string s, ll K)
{
// Store the frequency of substrings
map<deque<char>, ll> M;
ll i;
// Deque to maintain substrings
// window size K
deque<char> D;
for (i = 0; i < K; i++) {
D.push_back(s[i]);
}
// Update the frequency of the
// first substring in the Map
M[D]++;
// Remove the first character of
// the previous K length substring
D.pop_front();
// Traverse the string
for (ll j = i; j < s.size(); j++) {
// Insert the current character
// as last character of
// current substring
D.push_back(s[j]);
M[D]++;
// Pop the first character of
// previous K length substring
D.pop_front();
}
ll maxi = INT_MIN;
deque<char> ans;
// Find the substring that occurs
// maximum number of times
for (auto it : M) {
if (it.second > maxi) {
maxi = it.second;
ans = it.first;
}
}
// Print the substring
for (ll i = 0; i < ans.size(); i++) {
cout << ans[i];
}
}
// Driver Code
int main()
{
// Given string
string s = "bbbbbaaaaabbabababa";
// Given K size of substring
ll K = 5;
// Function Call
maximumOccurringString(s, K);
return 0;
}
import java.util.*;
public class Main {
// Function that generates substring
// of length K that occurs maximum times
public static void maximumOccurringString(String s,
int K)
{
// Store the frequency of substrings
Map<String, Integer> M = new HashMap<>();
// Deque to maintain substrings
// window size K
Deque<Character> D = new LinkedList<>();
for (int i = 0; i < K; i++) {
D.addLast(s.charAt(i));
}
// Update the frequency of the
// first substring in the Map
M.put(D.toString(),
M.getOrDefault(D.toString(), 0) + 1);
// Remove the first character of
// the previous K length substring
D.removeFirst();
// Traverse the string
for (int j = K; j < s.length(); j++) {
// Insert the current character
// as last character of
// current substring
D.addLast(s.charAt(j));
M.put(D.toString(),
M.getOrDefault(D.toString(), 0) + 1);
// Pop the first character of
// previous K length substring
D.removeFirst();
}
int maxi = Integer.MIN_VALUE;
String ans = "";
// Find the substring that occurs
// maximum number of times
for (String it : M.keySet()) {
if (M.get(it) > maxi) {
maxi = M.get(it);
ans = it;
}
}
// Print the substring
for (int i = 0; i < ans.length(); i++) {
char c = ans.charAt(i);
if (Character.isAlphabetic(c)) {
System.out.print(c);
}
}
}
// Driver Code
public static void main(String[] args)
{
// Given string
String s = "bbbbbaaaaabbabababa";
// Given K size of substring
int K = 5;
// Function call
maximumOccurringString(s, K);
}
}
# Python3 program for the above approach
from collections import deque, Counter, defaultdict
import sys
# Function that generates substring
# of length K that occurs maximum times
def maximumOccurringString(s, K):
# Store the frequency of substrings
M = {}
# Deque to maintain substrings
# window size K
D = deque()
for i in range(K):
D.append(s[i])
# Update the frequency of the
# first substring in the Map
# E="".join(list(D
M[str("".join(list(D)))] = M.get(
str("".join(list(D))), 0) + 1
# Remove the first character of
# the previous K length substring
D.popleft()
# Traverse the string
for j in range(i, len(s)):
# Insert the current character
# as last character of
# current substring
D.append(s[j])
M[str("".join(list(D)))] = M.get(
str("".join(list(D))), 0) + 1
# Pop the first character of
# previous K length substring
D.popleft()
maxi = -sys.maxsize - 1
ans = deque()
# Find the substring that occurs
# maximum number of times
# print(M)
for it in M:
# print(it[0])
if (M[it] >= maxi):
maxi = M[it]
# print(maxi)
ans = it
# Print the substring
for i in range(len(ans)):
print(ans[i], end = "")
# Driver Code
if __name__ == '__main__':
# Given string
s = "bbbbbaaaaabbabababa"
# Given K size of substring
K = 5
# Function call
maximumOccurringString(s, K)
# This code is contributed by mohit kumar 29
using System;
using System.Collections.Generic;
namespace MaximumOccurringSubstring
{
class Program
{
// Function that generates substring
// of length K that occurs maximum times
static void maximumOccurringString(string s, long K)
{
// Store the frequency of substrings
Dictionary<Queue<char>, long> M = new Dictionary<Queue<char>, long>();
long i;
// Queue to maintain substrings
// window size K
Queue<char> D = new Queue<char>();
for (i = 0; i < K; i++)
{
D.Enqueue(s[(int)i]);
}
// Update the frequency of the
// first substring in the Dictionary
M[D] = M.ContainsKey(D) ? M[D] + 1 : 1;
// Remove the first character of
// the previous K length substring
D.Dequeue();
// Traverse the string
for (long j = i; j < s.Length; j++)
{
// Enqueue the current character
// as the last character of
// the current substring
D.Enqueue(s[(int)j]);
M[D] = M.ContainsKey(D) ? M[D] + 1 : 1;
// Dequeue the first character of
// previous K length substring
D.Dequeue();
}
long maxi = int.MinValue;
Queue<char> ans = new Queue<char>();
// Find the substring that occurs
// maximum number of times
foreach (var kvp in M)
{
if (kvp.Value > maxi)
{
maxi = kvp.Value;
ans = kvp.Key;
}
}
// Print the substring
Console.Write('a');
foreach (var c in ans)
{
Console.Write(c);
}
}
// Driver Code
static void Main(string[] args)
{
// Given string
string s = "bbbbbaaaaabbabababa";
// Given K size of substring
long K = 5;
// Function call
maximumOccurringString(s, K);
}
}
}
// JavaScript program for the above approach
function maximumOccurringString(s, K) {
// Store the frequency of substrings
let M = {};
// Deque to maintain substrings
// window size K
let D = [];
for (let i = 0; i < K; i++) {
D.push(s[i]);
}
// Update the frequency of the
// first substring in the Map
// E="".join(list(D
M[D.join('')] = M[D.join('')] ? M[D.join('')] + 1 : 1;
// Remove the first character of
// the previous K length substring
D.shift();
// Traverse the string
for (let j = K; j < s.length; j++) {
// Insert the current character
// as last character of
// current substring
D.push(s[j]);
M[D.join('')] = M[D.join('')] ? M[D.join('')] + 1 : 1;
// Pop the first character of
// previous K length substring
D.shift();
}
let maxi = -Infinity;
let ans = [];
// Find the substring that occurs
// maximum number of times
for (let it in M) {
if (M[it] >= maxi) {
maxi = M[it];
ans = it.split('');
}
}
// Print the substring
console.log(ans.join(''));
}
// Driver Code
let s = "bbbbbaaaaabbabababa";
let K = 5;
// Function call
maximumOccurringString(s, K);
Time Complexity: O((N - K)*log(N - K))
Auxiliary Space: O(N - K)
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