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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 −
#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
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