Check if a string can be repeated to make another string
Last Updated :
07 May, 2025
Given two strings a and b, the task is to check how many times the string a can be repeated to generate the string b. If b cannot be generated by repeating a then print -1.
Examples:
Input: a = "geeks", b = "geeksgeeks"
Output: 2
"geeks" can be repeated twice to generate "geeksgeeks"
Input: a = "df", b = "dfgrt"
Output: -1
Approach:
- If len(b) % len(a) != 0 then print -1 as b cannot be generated by repeating a.
- Else set count = len(b) / len(a) and repeat a count number of times.
- If a = b then print count.
- Else print -1.
Below is the implementation of the above approach:
C++
// CPP implementation of the approach
#include<bits/stdc++.h>
using namespace std;
// Function to return the count of repetitions
// of string a to generate string b
int getCount(string a, string b)
{
// If b cannot be generated by repeating a
if(b.length() % a.length() != 0)
return -1;
int count = b.length() /a.length();
// Repeat a count number of times
string str = "";
for(int i = 0; i < count; i++)
{
str = str + a;
}
if(str == b)
return count;
return -1;
}
// Driver code
int main()
{
string a = "geeks";
string b = "geeksgeeks";
cout << (getCount(a, b));
return 0;
}
// This code is contributed by
// Surendra_Gangwar
// Java implementation of the approach
class GfG
{
// Function to return the count of repetitions
// of string a to generate string b
static int getCount(String a, String b)
{
// If b cannot be generated by repeating a
if(b.length() % a.length() != 0)
return -1;
int count = b.length() / a.length();
// Repeat a count number of times
String str = "";
for(int i = 0; i < count; i++)
{
str = str + a;
}
if(str.equals(b))
return count;
return -1;
}
// Driver code
public static void main(String []args)
{
String a = "geeks";
String b = "geeksgeeks";
System.out.println(getCount(a, b));
}
}
// This code is contributed by Rituraj Jain
# Python3 implementation of the approach
# Function to return the count of repetitions
# of string a to generate string b
def getCount(a, b):
# If b cannot be generated by repeating a
if(len(b) % len(a) != 0):
return -1;
count = int(len(b) / len(a))
# Repeat a count number of times
a = a * count
if(a == b):
return count
return -1;
# Driver code
if __name__ == '__main__':
a = 'geeks'
b = 'geeksgeeks'
print(getCount(a, b))
// C# implementation of the approach
using System;
class GfG
{
// Function to return the count of repetitions
// of string a to generate string b
static int getCount(String a, String b)
{
// If b cannot be generated by repeating a
if(b.Length % a.Length != 0)
return -1;
int count = b.Length / a.Length;
// Repeat a count number of times
String str = "";
for(int i = 0; i < count; i++)
{
str = str + a;
}
if(str.Equals(b))
return count;
return -1;
}
// Driver code
public static void Main(String []args)
{
String a = "geeks";
String b = "geeksgeeks";
Console.WriteLine(getCount(a, b));
}
}
// This code contributed by Rajput-Ji
<script>
// Javascript implementation of the approach
// Function to return the count of repetitions
// of string a to generate string b
function getCount( a, b) {
// If b cannot be generated by repeating a
if (b.length % a.length != 0)
return -1;
var count = parseInt(b.length / a.length);
// Repeat a count number of times
var str = "";
for (i = 0; i < count; i++) {
str = str + a;
}
if (str == (b))
return count;
return -1;
}
// Driver code
var a = "geeks";
var b = "geeksgeeks";
document.write(getCount(a, b));
// This code is contributed by todaysgaurav
</script>
<?php
// PHP implementation of the approach
// Function to return the count of repetitions
// of string a to generate string b
function getCount($a, $b)
{
// If b cannot be generated by repeating a
if(strlen($b) % strlen($a) != 0)
return -1;
$count = floor(strlen($b) / strlen($a));
// Repeat a count number of times
// Repeat a count number of times
$str = "";
for($i = 0; $i < $count; $i++)
{
$str = $str . $a ;
}
if(strcmp($a,$b))
return $count;
return -1;
}
// Driver code
$a = 'geeks';
$b = 'geeksgeeks';
echo getCount($a, $b);
// This code is contributed by Ryuga
?>
Time Complexity: O(n/m), where m and n are the lengths of the given strings a and b respectively.
Auxiliary Space: O(n), where n is the length of the string.
Approach:
- Check if the length of string b is a multiple of the length of string a. If not, return -1.
- Calculate the number of repetitions of a needed to generate b.
- Create an empty stack.
- Push each substring of length n (i.e., the length of a) from b onto the stack. This is done by iterating from 0 to the number of repetitions needed, and calling the substr() function on b with the starting index i*n and length n.
- Pop each substring from the stack and append it to a new string str. This is done by iterating until the stack is empty, calling the top() function to get the top element of the stack, concatenating it to str, and then calling the pop() function to remove the top element.
- Check if str is equal to b. If it is, return the number of repetitions calculated in step 2; otherwise, return -1.
Below is the implementation of the above approach:
C++
#include <iostream>
#include <stack>
using namespace std;
int count_repetitions(string a, string b) {
int n = a.length();
int m = b.length();
if (m % n != 0) {
return -1;
}
int repetitions = m / n;
stack<string> s;
for (int i = 0; i < repetitions; i++) {
s.push(b.substr(i*n, n));
}
string str = "";
while (!s.empty()) {
str += s.top();
s.pop();
}
if (str == b) {
return repetitions;
} else {
return -1;
}
}
int main() {
string a = "geeks";
string b = "geeksgeeks";
cout << count_repetitions(a, b) << endl;
return 0;
}
import java.util.*;
public class Main {
public static void main(String[] args) {
String a = "geeks";
String b = "geeksgeeks";
System.out.println(countRepetitions(a, b));
}
// Function to count the number of times string 'a' is repeated to form string 'b'
public static int countRepetitions(String a, String b) {
int n = a.length();
int m = b.length();
// If the length of string 'b' is not a multiple of the length of string 'a', it's not possible to repeat 'a' to form 'b'
if (m % n != 0) {
return -1;
}
// Calculate the number of repetitions required to form 'b' from 'a'
int repetitions = m / n;
// Use a stack to store the individual substrings of 'b' that correspond to each repetition of 'a'
Stack<String> stack = new Stack<>();
for (int i = 0; i < repetitions; i++) {
// Push each substring into the stack by extracting it from 'b'
stack.push(b.substring(i * n, (i + 1) * n));
}
// Build a new string by popping the substrings from the stack in reverse order
StringBuilder str = new StringBuilder();
while (!stack.isEmpty()) {
str.append(stack.pop());
}
// If the final concatenated string matches 'b', it means 'a' is repeated 'repetitions' times to form 'b'
// Return the number of repetitions, else return -1
if (str.toString().equals(b)) {
return repetitions;
} else {
return -1;
}
}
}
def count_repetitions(a, b):
n = len(a)
m = len(b)
# Check if the length of 'b' is a multiple of the length of 'a'
if m % n != 0:
return -1
repetitions = m // n
stack = []
# Split 'b' into 'repetitions' parts and store them in the stack
for i in range(repetitions):
stack.append(b[i * n: (i + 1) * n])
str_result = ""
# Pop elements from the stack and append them to 'str_result'
while stack:
str_result += stack.pop()
# Compare 'str_result' with 'b' to check if they are the same
if str_result == b:
return repetitions
else:
return -1
if __name__ == "__main__":
a = "geeks"
b = "geeksgeeks"
print(count_repetitions(a, b))
# This code is contributed by shivamgupta0987654321
using System;
public class GFG
{
// Function to count repetitions of 'a' in 'b'
public static int CountRepetitions(string a, string b)
{
int n = a.Length;
int m = b.Length;
// Check if the length of 'b' is a multiple of the length of 'a'
if (m % n != 0)
{
return -1;
}
int repetitions = m / n;
var stack = new System.Collections.Generic.Stack<string>();
// Split 'b' into 'repetitions' parts and store them in the stack
for (int i = 0; i < repetitions; i++)
{
stack.Push(b.Substring(i * n, n));
}
string strResult = "";
// Pop elements from the stack and append them to 'strResult'
while (stack.Count > 0)
{
strResult += stack.Pop();
}
// Compare 'strResult' with 'b' to check if they are the same
if (strResult == b)
{
return repetitions;
}
else
{
return -1;
}
}
public static void Main(string[] args)
{
string a = "geeks";
string b = "geeksgeeks";
Console.WriteLine(CountRepetitions(a, b));
}
}
// This code is contributed by rambabuguphka
function countRepetitions(a, b) {
const n = a.length;
const m = b.length;
// Check if the length of 'b' is a multiple of the length of 'a'
if (m % n !== 0) {
return -1;
}
const repetitions = m / n;
const stack = [];
// Split 'b' into 'repetitions' parts and store them in the stack
for (let i = 0; i < repetitions; i++) {
stack.push(b.substring(i * n, (i + 1) * n));
}
let strResult = "";
// Pop elements from the stack and append them to 'strResult'
while (stack.length > 0) {
strResult += stack.pop();
}
// Compare 'strResult' with 'b' to check if they are the same
if (strResult === b) {
return repetitions;
} else {
return -1;
}
}
const a = "geeks";
const b = "geeksgeeks";
console.log(countRepetitions(a, b));
// This code is contributed by shivamgupta0987654321
Time Complexity: O(m), where m is the length of the string b.
Auxiliary Space: O(m/n + m), where n is the length of the string a and m is the length of the string b.
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