Program to find longest prefix that is also a suffix in C++

Suppose we have a string s, we have to find the longest prefix of s, which is also a suffix (excluding itself). If there is no such prefix, then simply return blank string.

So, if the input is like "madam", then the output will be "m", it has 4 prefixes excluding itself. These are "m", "ma", "mad", "mada" and 4 suffixes like "m", "am", "dam", "adam". The largest prefix which is also suffix is given by "m".

To solve this, we will follow these steps −

Define a function lps(), this will take s,
n := size of s
Define an array ret of size n
j := 0, i := 1
while i < n, do −
- if s[i] is same as s[j], then −
  - ret[i] := j + 1
  - (increase i by 1)
  - (increase j by 1)
- otherwise when s[i] is not equal to s[j], then −
  - if j > 0, then −
    - j := ret[j − 1]
  - Otherwise
    - (increase i by 1)
return ret
From the main method do the following −
n := size of s
if n is same as 1, then −
- return blank string
Define an array v = lps(s)
x := v[n − 1]
ret := blank string
for initialize i := 0, when i < x, update (increase i by 1), do −
- ret := ret + s[i]
return ret

Let us see the following implementation to get better understanding −

Example

Live Demo

#include <bits/stdc++.h>
using namespace std;
class Solution {
   public:
   vector <int> lps(string s){
      int n = s.size();
      vector<int> ret(n);
      int j = 0;
      int i = 1;
      while (i < n) {
         if (s[i] == s[j]) {
            ret[i] = j + 1;
            i++;
            j++;
         }
         else if (s[i] != s[j]) {
            if (j > 0)
            j = ret[j - 1];
            else {
               i++;
            }
         }
      }
      return ret;
   }
   string longestPrefix(string s) {
      int n = s.size();
      if (n == 1)
      return "";
      vector<int> v = lps(s);
      int x = v[n - 1];
      string ret = "";
      for (int i = 0; i < x; i++) {
         ret += s[i];
      }
      return ret;
   }
};
main(){
   Solution ob;
   cout << (ob.longestPrefix("helloworldhello"));
}