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C++ Program to Check if strings are rotations of each other or not | Set 2

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Given two strings s1 and s2, check whether s2 is a rotation of s1. 

Output : True

Output : True

We have discussed an approach in earlier post which handles substring match as a pattern. In this post, we will be going to use KMP algorithm’s lps (longest proper prefix which is also suffix) construction, which will help in finding the longest match of the prefix of string b and suffix of string a. By which we will know the rotating point, from this point match the characters. If all the characters are matched, then it is a rotation, else not.
Below is the basic implementation of the above approach. 


// C++ program to check if 
// two strings are rotations
// of each other
using namespace std;
bool isRotation(string a, 
                string b)
  int n = a.length();
  int m = b.length();
  if (n != m)
    return false;
  // create lps[] that 
  // will hold the longest
  // prefix suffix values 
  // for pattern
  int lps[n];
  // length of the previous 
  // longest prefix suffix
  int len = 0;
  int i = 1;
  // lps[0] is always 0
  lps[0] = 0; 
  // the loop calculates 
  // lps[i] for i = 1 to n-1
  while (i < n) 
    if (a[i] == b[len]) 
      lps[i] = ++len;
      if (len == 0) 
        lps[i] = 0;
        len = lps[len - 1];
  i = 0;
  // Match from that rotating
  // point
  for (int k = lps[n - 1]; 
           k < m; ++k) 
    if (b[k] != a[i++])
      return false;
  return true;
// Driver code
int main()
  string s1 = "ABACD";
  string s2 = "CDABA";
  cout << (isRotation(s1, s2) ? 
           "1" : "0");
// This code is contributed by Chitranayal



Time Complexity: O(n) 
Auxiliary Space: O(n)

Please refer complete article on Check if strings are rotations of each other or not | Set 2 for more details!

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Last Updated : 24 Jan, 2022
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