Minimum Characters to be Added at Front to Make String Palindrome


Difficulty Level Medium
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String

Problem Statement

In the “Minimum Characters to be Added at Front to Make String Palindrome” problem we have given a string “s”. Write a program to find the minimum characters to be added at the front to make a string palindrome.

Input Format

The first and only one line containing a string “s”.

Output Format

The first and only one line containing an integer value n. Here n denotes the minimum characters to be added at the front to make a string palindrome.

Constraints

  • 1<=|s|<=10^6
  • s[i] must be a lower case English alphabet

Example

edef
1

Explanation: If we add “f” at the front of string “s” then our string satisfied the condition of the palindrome. So, here only 1 should be added in the front.

Algorithm

1. Concatenate the string with a special symbol and reverse of the given string ie, c = s + $ + rs

2. Build an LPS array in which each index represents the longest proper prefix which is also a suffix.

3. As the last value in the LPS array is the largest suffix that matches the prefix of the original string. So, there are those many characters that satisfy the palindrome property

4. Now, find the difference between the length of the string and the last value in the LPS array, which is the minimum number of characters needed to make a string palindrome.

Working of above example

s = “edef”

Example for proper prefixes and suffix(Knowledge required to form LPS)

Proper prefixes of “abc” areĀ  ” “, “a”, “ab” and Suffixes of “abc” are “c”, “bc”, “abc”.

s (after concatenation)= “edef$fede”

LPS = {0, 0, 1, 0, 0, 0, 1, 2, 3 } //Here index is the longest length of largest prefix that matches suffix.

The last value of LPS = 3, so there are 3 characters that satisfy palindrome property.

Now, find the difference between the length of a given string(ie, 4) and the Last value(3)

Therefore, we need 1 character to make it a palindrome.

Implementation

C++ program for Minimum Characters to be Added at Front to Make String Palindrome

#include <bits/stdc++.h>
using namespace std;
 
vector<int> computeLPSArray(string s)
{
    int n = s.length();
    vector<int> LPS(n);
 
    int len = 0;
    LPS[0] = 0; 
    int i = 1;
    while (i < n)
    {
        if (s[i] == s[len])
        {
            len++;
            LPS[i] = len;
            i++;
        }
        else
        {
            if (len != 0)
            {
                len = LPS[len-1];
            }
            else 
            {
                LPS[i] = 0;
                i++;
            }
        }
    }
    return LPS;
}

int solve(string s)
{
    string rs = s;
    reverse(rs.begin(), rs.end());
    string c = s + "$" + rs;
    vector<int> LPS = computeLPSArray(c);
    return (s.length() - LPS.back());
}
 
int main()
{
    string s;
    cin>>s;
    cout<<solve(s)<<endl;
    return 0;
}

Java program for Minimum Characters to be Added at Front to Make String Palindrome

import static java.lang.Math.abs;
import java.util.Scanner;

class sum
{ 
        public static int[] computeLPSArray(String str) 
        { 
            int n = str.length(); 
            int lps[] = new int[n]; 
            int i = 1, len = 0; 
            lps[0] = 0;
            while (i < n)  
            { 
                if (str.charAt(i) == str.charAt(len)) 
                { 
                    len++; 
                    lps[i] = len; 
                    i++; 
                } 
                else
                { 
                    if (len != 0) 
                    { 
                        len = lps[len - 1];  
                    } 
                    else
                    { 
                        lps[i] = 0; 
                        i++; 
                    } 
                } 
            } 
            return lps; 
        } 
        static int solve(String str)  
        {  
            StringBuilder s = new StringBuilder(); 
            s.append(str); 
            String rev = s.reverse().toString(); 
            s.reverse().append("$").append(rev); 
            int lps[] = computeLPSArray(s.toString()); 
            return str.length() - lps[s.length() - 1]; 
        } 

  public static void main(String[] args) 
  { 
    Scanner sr = new Scanner(System.in); 
                String s = sr.next();
                System.out.println(solve(s));
  } 
} 
qsjhbqd
6

Complexity Analysis

Time Complexity

O(n) where n is the size of the given string “s”. Here we find the “lps array of KMP algorithm” which takes linear time to compute.

Space Complexity

O(n) because we create an LSP array to compute our answer. And here the maximum size of the LSP array is n.

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