Shortest Palindrome in C++

Suppose we have a string s. We can convert it to palindrome by adding characters in front of it. We have to find the shortest palindrome, that we can find performing this information. So if the string is like “abcc”, then the result will be − "ccbabcc".

To solve this, we will follow these steps −

n := size of s, s1 := s, s2 := s
Reverse the string s2
s2 := s concatenate "#" concatenate s2
Define an array lps of size same as s2
j := 0, i := 1
while i < size of s2, do −
- if s2[i] is same as s2[j], then,
  - lps[i] := j + 1
  - increase i by 1, increase j by 1
- Otherwise
  - if j > 0, then, j := lps[j - 1]
  - Otherwise increase i by 1
extra := substring of s from lps[size of s – 1] to n - lps[size of lps - 1])
Reverse extra
return extra concatenate s

Example

Let us see the following implementation to get better understanding −

Live Demo

#include <bits/stdc++.h>
using namespace std;
class Solution {
   public:
   string shortestPalindrome(string s) {
      int n = s.size();
      string s1 = s;
      string s2 = s;
      reverse(s2.begin(), s2.end());
      s2 = s + "#" + s2;
      vector <int> lps(s2.size());
      int j = 0;
      int i = 1;
      while(i <s2.size()){
         if(s2[i] == s2[j]){
            lps[i] = j + 1;
            j++;
            i++;
         } else {
            if(j > 0){
               j = lps[ j - 1];
            } else {
               i++;
            }
         }
      }
      string extra = s.substr(lps[lps.size() - 1], n - lps[lps.size() - 1]);
      reverse(extra.begin(), extra.end());
      return extra + s;
   }
};
main(){
   Solution ob;
   cout << (ob.shortestPalindrome("abcc"));
}

Input

“abcc”

Output

ccbabcc

Arnab Chakraborty

Published on 26-May-2020 14:05:29

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