Minimum insertions to form a palindrome

Given a string str, the task is to find the minimum number of characters to be inserted to convert it to palindrome.

Before we go further, let us understand with few examples:

ab: Number of insertions required is 1 i.e. bab
aa: Number of insertions required is 0 i.e. aa
abcd: Number of insertions required is 3 i.e. dcbabcd
abcda: Number of insertions required is 2 i.e. adcbcda which is same as number of insertions in the substring bcd(Why?).
abcde: Number of insertions required is 4 i.e. edcbabcde

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

Let the input string be str[l……h]. The problem can be broken down into three parts:

Find the minimum number of insertions in the substring str[l+1,…….h].
Find the minimum number of insertions in the substring str[l…….h-1].
Find the minimum number of insertions in the substring str[l+1……h-1].

Recursive Approach: The minimum number of insertions in the string str[l…..h] can be given as:

minInsertions(str[l+1…..h-1]) if str[l] is equal to str[h]
min(minInsertions(str[l…..h-1]), minInsertions(str[l+1…..h])) + 1 otherwise

Below is the implementation of the above approach:

C++

// A Naive recursive program to find minimum  
// number insertions needed to make a string 
// palindrome 
#include<bits/stdc++.h> 
using namespace std; 
  
  
// Recursive function to find   
// minimum number of insertions 
int findMinInsertions(char str[], int l, int h) 
{ 
    // Base Cases 
    if (l > h) return INT_MAX; 
    if (l == h) return 0; 
    if (l == h - 1) return (str[l] == str[h])? 0 : 1; 
  
    // Check if the first and last characters are 
    // same. On the basis of the comparison result,  
    // decide which subrpoblem(s) to call 
    return (str[l] == str[h])?  
                    findMinInsertions(str, l + 1, h - 1): 
                    (min(findMinInsertions(str, l, h - 1), 
                    findMinInsertions(str, l + 1, h)) + 1); 
} 
  
// Driver code 
int main() 
{ 
    char str[] = "geeks"; 
    cout << findMinInsertions(str, 0, strlen(str) - 1); 
    return 0; 
} 
  
// This code is contributed by  
// Akanksha Rai 

C

// A Naive recursive program to find minimum  
// number insertions needed to make a string 
// palindrome 
#include <stdio.h> 
#include <limits.h> 
#include <string.h> 
  
// A utility function to find minimum of two numbers 
int min(int a, int b) 
{  return a < b ? a : b; } 
  
// Recursive function to find minimum number of  
// insertions 
int findMinInsertions(char str[], int l, int h) 
{ 
    // Base Cases 
    if (l > h) return INT_MAX; 
    if (l == h) return 0; 
    if (l == h - 1) return (str[l] == str[h])? 0 : 1; 
  
    // Check if the first and last characters are 
    // same. On the basis of the comparison result,  
    // decide which subrpoblem(s) to call 
    return (str[l] == str[h])?  
                     findMinInsertions(str, l + 1, h - 1): 
                     (min(findMinInsertions(str, l, h - 1), 
                     findMinInsertions(str, l + 1, h)) + 1); 
} 
  
// Driver program to test above functions 
int main() 
{ 
    char str[] = "geeks"; 
    printf("%d", findMinInsertions(str, 0, strlen(str)-1)); 
    return 0; 
} 

Java

// A Naive recursive Java program to find minimum 
// number insertions needed to make a string 
// palindrome 
class GFG { 
  
    // Recursive function to find minimum number 
    // of insertions 
    static int findMinInsertions(char str[], int l, 
                                             int h) 
    { 
        // Base Cases 
        if (l > h) return Integer.MAX_VALUE; 
        if (l == h) return 0; 
        if (l == h - 1) return (str[l] == str[h])? 0 : 1; 
  
        // Check if the first and last characters 
        // are same. On the basis of the  comparison 
        // result, decide which subrpoblem(s) to call 
        return (str[l] == str[h])? 
            findMinInsertions(str, l + 1, h - 1): 
            (Integer.min(findMinInsertions(str, l, h - 1), 
            findMinInsertions(str, l + 1, h)) + 1); 
    } 
  
    // Driver program to test above functions 
    public static void main(String args[]) 
    { 
        String str= "geeks"; 
        System.out.println(findMinInsertions(str.toCharArray(), 
                                          0, str.length()-1)); 
    } 
} 
// This code is contributed by Sumit Ghosh 

Python 3

# A Naive recursive program to find minimum  
# number insertions needed to make a string 
# palindrome 
import sys 
  
# Recursive function to find minimum  
# number of insertions 
def findMinInsertions(str, l, h): 
  
    # Base Cases 
    if (l > h): 
        return sys.maxsize 
    if (l == h): 
        return 0
    if (l == h - 1): 
        return 0 if(str[l] == str[h]) else 1
  
    # Check if the first and last characters are 
    # same. On the basis of the comparison result,  
    # decide which subrpoblem(s) to call 
      
    if(str[l] == str[h]): 
        return findMinInsertions(str, l + 1, h - 1) 
    else: 
        return (min(findMinInsertions(str, l, h - 1), 
                    findMinInsertions(str, l + 1, h)) + 1) 
  
# Driver Code 
if __name__ == "__main__": 
      
    str = "geeks"
    print(findMinInsertions(str, 0, len(str) - 1)) 
  
# This code is contributed by ita_c 

C#

// A Naive recursive C# program  
// to find minimum number  
// insertions needed to make  
// a string palindrom 
using System; 
  
class GFG 
{ 
    // Recursive function to  
    // find minimum number of 
    // insertions 
    static int findMinInsertions(char []str,  
                                 int l, int h) 
    { 
        // Base Cases 
        if (l > h) return int.MaxValue; 
        if (l == h) return 0; 
        if (l == h - 1)  
            return (str[l] == str[h])? 0 : 1; 
  
        // Check if the first and  
        // last characters are same.  
        // On the basis of the  
        // comparison result, decide  
        // which subrpoblem(s) to call 
        return (str[l] == str[h])? 
                findMinInsertions(str,  
                                  l + 1, h - 1): 
                (Math.Min(findMinInsertions(str, l,  
                                            h - 1), 
                          findMinInsertions(str, l +  
                                        1, h)) + 1); 
    }  
      
    // Driver Code 
    public static void Main() 
    { 
        string str= "geeks"; 
        Console.WriteLine(findMinInsertions(str.ToCharArray(), 
                                            0, str.Length - 1));  
    } 
} 
  
// This code is contributed by Sam007 

Output:

Dynamic Programming based Solution
If we observe the above approach carefully, we can find that it exhibits overlapping subproblems.
Suppose we want to find the minimum number of insertions in string “abcde”:

                      abcde
            /       |      \
           /        |        \
           bcde         abcd       bcd  <- case 3 is discarded as str[l] != str[h]
       /   |   \       /   |   \
      /    |    \     /    |    \
     cde   bcd  cd   bcd abc bc
   / | \  / | \ /|\ / | \
de cd d cd bc c………………….

The substrings in bold show that the recursion to be terminated and the recursion tree cannot originate from there. Substring in the same color indicates overlapping subproblems.

How to re-use solutions of subproblems? Memoization technique is used to avoid similar subproblem recalls. We can create a table to store results of subproblems so that they can be used directly if same subproblem is encountered again.

The below table represents the stored values for the string abcde.

a b c d e
----------
0 1 2 3 4
0 0 1 2 3 
0 0 0 1 2 
0 0 0 0 1 
0 0 0 0 0

How to fill the table?
The table should be filled in diagonal fashion. For the string abcde, 0….4, the following should be order in which the table is filled:

Gap = 1: (0, 1) (1, 2) (2, 3) (3, 4)

Gap = 2: (0, 2) (1, 3) (2, 4)

Gap = 3: (0, 3) (1, 4)

Gap = 4: (0, 4)

Below is the implementation of the above approach:

C++

// A Dynamic Programming based program to find  
// minimum number insertions needed to make a  
// string palindrome  
#include <bits/stdc++.h> 
using namespace std; 
  
  
// A DP function to find minimum 
// number of insertions  
int findMinInsertionsDP(char str[], int n)  
{  
    // Create a table of size n*n. table[i][j]  
    // will store minimum number of insertions  
    // needed to convert str[i..j] to a palindrome.  
    int table[n][n], l, h, gap;  
  
    // Initialize all table entries as 0  
    memset(table, 0, sizeof(table));  
  
    // Fill the table  
    for (gap = 1; gap < n; ++gap)  
        for (l = 0, h = gap; h < n; ++l, ++h)  
            table[l][h] = (str[l] == str[h])?  
                        table[l + 1][h - 1] :  
                        (min(table[l][h - 1],  
                             table[l + 1][h]) + 1);  
  
    // Return minimum number of insertions 
    // for str[0..n-1]  
    return table[0][n - 1];  
}  
  
// Driver Code 
int main()  
{  
    char str[] = "geeks";  
    cout << findMinInsertionsDP(str, strlen(str));  
    return 0;  
}  
  
// This is code is contributed by rathbhupendra 

C

// A Dynamic Programming based program to find 
// minimum number insertions needed to make a 
// string palindrome 
#include <stdio.h> 
#include <string.h> 
  
// A utility function to find minimum of two integers 
int min(int a, int b) 
{   return a < b ? a : b;  } 
  
// A DP function to find minimum number of insertions 
int findMinInsertionsDP(char str[], int n) 
{ 
    // Create a table of size n*n. table[i][j] 
    // will store minimum number of insertions  
    // needed to convert str[i..j] to a palindrome. 
    int table[n][n], l, h, gap; 
  
    // Initialize all table entries as 0 
    memset(table, 0, sizeof(table)); 
  
    // Fill the table 
    for (gap = 1; gap < n; ++gap) 
        for (l = 0, h = gap; h < n; ++l, ++h) 
            table[l][h] = (str[l] == str[h])? 
                          table[l+1][h-1] : 
                          (min(table[l][h-1],  
                           table[l+1][h]) + 1); 
  
    // Return minimum number of insertions for 
    // str[0..n-1] 
    return table[0][n-1]; 
} 
  
// Driver program to test above function. 
int main() 
{ 
    char str[] = "geeks"; 
    printf("%d", findMinInsertionsDP(str, strlen(str))); 
    return 0; 
} 

Java

// A Java solution for Dynamic Programming 
// based program to find minimum number 
// insertions needed to make a string 
// palindrome 
import java.util.Arrays; 
  
class GFG 
{ 
    // A DP function to find minimum number 
    // of insersions 
    static int findMinInsertionsDP(char str[], int n) 
    { 
        // Create a table of size n*n. table[i][j] 
        // will store minumum number of insertions 
        // needed to convert str[i..j] to a palindrome. 
        int table[][] = new int[n][n]; 
        int l, h, gap; 
  
        // Fill the table 
        for (gap = 1; gap < n; ++gap) 
        for (l = 0, h = gap; h < n; ++l, ++h) 
            table[l][h] = (str[l] == str[h])? 
                           table[l+1][h-1] : 
                          (Integer.min(table[l][h-1], 
                                 table[l+1][h]) + 1); 
  
        // Return minimum number of insertions 
        // for str[0..n-1] 
        return table[0][n-1]; 
    } 
  
    // Driver program to test above function. 
    public static void main(String args[]) 
    { 
        String str = "geeks"; 
        System.out.println( 
        findMinInsertionsDP(str.toCharArray(), str.length())); 
    } 
} 
// This code is contributed by Sumit Ghosh 

Python3

# A Dynamic Programming based program to  
# find minimum number insertions needed  
# to make a str1ing palindrome 
  
# A utility function to find minimum  
# of two integers 
def Min(a, b): 
    return min(a, b) 
  
# A DP function to find minimum number 
# of insertions 
def findMinInsertionsDP(str1, n): 
  
    # Create a table of size n*n. table[i][j] 
    # will store minimum number of insertions  
    # needed to convert str1[i..j] to a palindrome. 
    table = [[0 for i in range(n)]  
                for i in range(n)] 
    l, h, gap = 0, 0, 0
  
    # Fill the table 
    for gap in range(1, n): 
        l = 0
        for h in range(gap, n): 
            if str1[l] == str1[h]: 
                table[l][h] = table[l + 1][h - 1] 
            else: 
                table[l][h] = (Min(table[l][h - 1],  
                                   table[l + 1][h]) + 1) 
            l += 1
  
    # Return minimum number of insertions  
    # for str1[0..n-1] 
    return table[0][n - 1]; 
  
# Driver Code 
str1 = "geeks"
print(findMinInsertionsDP(str1, len(str1))) 
  
# This code is contributed by  
# Mohit kumar 29 

C#

// A C# solution for Dynamic Programming 
// based program to find minimum number 
// insertions needed to make a string 
// palindrome 
using System; 
  
class GFG 
{ 
    // A DP function to find minimum number 
    // of insersions 
    static int findMinInsertionsDP(char []str, int n) 
    { 
        // Create a table of size n*n. table[i][j] 
        // will store minumum number of insertions 
        // needed to convert str[i..j] to a palindrome. 
        int [,]table = new int[n, n]; 
        int l, h, gap; 
  
        // Fill the table 
        for (gap = 1; gap < n; ++gap) 
        for (l = 0, h = gap; h < n; ++l, ++h) 
            table[l, h] = (str[l] == str[h])? 
                        table[l+1, h-1] : 
                        (Math.Min(table[l, h-1], 
                                table[l+1, h]) + 1); 
  
        // Return minimum number of insertions 
        // for str[0..n-1] 
        return table[0, n-1]; 
    } 
  
    // Driver code 
    public static void Main() 
    { 
        String str = "geeks"; 
        Console.Write( 
        findMinInsertionsDP(str.ToCharArray(), str.Length)); 
    } 
} 
  
// This code is contributed by Rajput-Ji 

Output:

Time complexity: O(N^2)
Auxiliary Space: O(N^2)

Another Dynamic Programming Solution (Variation of Longest Common Subsequence Problem)
The problem of finding minimum insertions can also be solved using Longest Common Subsequence (LCS) Problem. If we find out LCS of string and its reverse, we know how many maximum characters can form a palindrome. We need insert remaining characters. Following are the steps.

Find the length of LCS of input string and its reverse. Let the length be ‘l’.
The minimum number insertions needed is length of input string minus ‘l’.

Below is the implementation of the above approach:

C++

// An LCS based program to find minimum number  
// insertions needed to make a string palindrome  
#include <bits/stdc++.h> 
using namespace std; 
   
  
// Returns length of LCS for X[0..m-1], Y[0..n-1].  
int lcs( string X, string Y, int m, int n )  
{  
    int L[m+1][n+1];  
    int i, j;  
      
    /* Following steps build L[m+1][n+1] in bottom  
        up fashion. Note that L[i][j] contains length  
        of LCS of X[0..i-1] and Y[0..j-1] */
    for (i = 0; i <= m; i++)  
    {  
        for (j = 0; j <= n; j++)  
        {  
        if (i == 0 || j == 0)  
            L[i][j] = 0;  
      
        else if (X[i - 1] == Y[j - 1])  
            L[i][j] = L[i - 1][j - 1] + 1;  
      
        else
            L[i][j] = max(L[i - 1][j], L[i][j - 1]);  
        }  
    }  
      
    /* L[m][n] contains length of LCS for X[0..n-1]  
        and Y[0..m-1] */
    return L[m][n];  
}  
  
void reverseStr(string& str)  
{  
    int n = str.length();  
  
    // Swap character starting from two  
    // corners  
    for (int i = 0; i < n / 2; i++)  
        swap(str[i], str[n - i - 1]);  
}  
  
// LCS based function to find minimum number of  
// insertions  
int findMinInsertionsLCS(string str, int n)  
{  
    // Creata another string to store reverse of 'str'  
    string rev = "";  
    rev = str;  
    reverseStr(rev);  
      
    // The output is length of string minus length of lcs of  
    // str and it reverse  
    return (n - lcs(str, rev, n, n));  
}  
  
// Driver code 
int main()  
{  
    string str = "geeks";  
    cout << findMinInsertionsLCS(str, str.length());  
    return 0;  
}  
  
// This code is contributed by rathbhupendra 

C

// An LCS based program to find minimum number 
// insertions needed to make a string palindrome 
#include<stdio.h> 
#include <string.h> 
  
/* Utility function to get max of 2 integers */
int max(int a, int b) 
{   return (a > b)? a : b; } 
  
/* Returns length of LCS for X[0..m-1], Y[0..n-1].  
   See http://goo.gl/bHQVP for details of this  
   function */
int lcs( char *X, char *Y, int m, int n ) 
{ 
   int L[m+1][n+1]; 
   int i, j; 
  
   /* Following steps build L[m+1][n+1] in bottom  
      up fashion. Note that L[i][j] contains length 
      of LCS of X[0..i-1] and Y[0..j-1] */
   for (i=0; i<=m; i++) 
   { 
     for (j=0; j<=n; j++) 
     { 
       if (i == 0 || j == 0) 
         L[i][j] = 0; 
  
       else if (X[i-1] == Y[j-1]) 
         L[i][j] = L[i-1][j-1] + 1; 
  
       else
         L[i][j] = max(L[i-1][j], L[i][j-1]); 
     } 
   } 
  
   /* L[m][n] contains length of LCS for X[0..n-1] 
     and Y[0..m-1] */
   return L[m][n]; 
} 
  
// LCS based function to find minimum number of  
// insertions 
int findMinInsertionsLCS(char str[], int n) 
{ 
   // Creata another string to store reverse of 'str' 
   char rev[n+1]; 
   strcpy(rev, str); 
   strrev(rev); 
  
   // The output is length of string minus length of lcs of 
   // str and it reverse 
   return (n - lcs(str, rev, n, n)); 
} 
  
// Driver program to test above functions 
int main() 
{ 
    char str[] = "geeks"; 
    printf("%d", findMinInsertionsLCS(str, strlen(str))); 
    return 0; 
} 

Java

// An LCS based Java program to find minimum 
// number insertions needed to make a string 
// palindrome 
class GFG 
{ 
    /* Returns length of LCS for X[0..m-1], 
       Y[0..n-1]. See http://goo.gl/bHQVP for 
       details of this function */
    static int lcs( String X, String Y, int m, int n ) 
    { 
        int L[][] = new int[m+1][n+1]; 
        int i, j; 
  
        /* Following steps build L[m+1][n+1] in 
           bottom up fashion. Note that L[i][j] 
           contains length of LCS of X[0..i-1] 
           and Y[0..j-1] */
        for (i=0; i<=m; i++) 
        { 
            for (j=0; j<=n; j++) 
            { 
                if (i == 0 || j == 0) 
                    L[i][j] = 0; 
  
                else if (X.charAt(i-1) == Y.charAt(j-1)) 
                    L[i][j] = L[i-1][j-1] + 1; 
  
                else
                    L[i][j] = Integer.max(L[i-1][j], L[i][j-1]); 
            } 
        } 
  
        /* L[m][n] contains length of LCS for 
           X[0..n-1] and Y[0..m-1] */
        return L[m][n]; 
    } 
  
    // LCS based function to find minimum number 
    // of insersions 
    static int findMinInsertionsLCS(String str, int n) 
    { 
        // Using StringBuffer to reverse a String 
        StringBuffer sb = new StringBuffer(str); 
        sb.reverse(); 
        String revString = sb.toString(); 
  
        // The output is length of string minus 
        // length of lcs of str and it reverse 
        return (n - lcs(str, revString , n, n)); 
    } 
  
    // Driver program to test above functions 
    public static void main(String args[]) 
    { 
        String str = "geeks"; 
        System.out.println( 
        findMinInsertionsLCS(str, str.length())); 
    } 
} 
// This code is contributed by Sumit Ghosh 

C#

// An LCS based C# program to find minimum 
// number insertions needed to make a string 
// palindrome 
using System; 
  
class GFG 
{ 
    /* Returns length of LCS for X[0..m-1], 
    Y[0..n-1]. See http://goo.gl/bHQVP for 
    details of this function */
    static int lcs( string X, string Y, int m, int n ) 
    { 
        int[,] L = new int[m + 1, n + 1]; 
        int i, j; 
  
        /* Following steps build L[m+1,n+1] in 
        bottom up fashion. Note that L[i,j] 
        contains length of LCS of X[0..i-1] 
        and Y[0..j-1] */
        for (i = 0; i <= m; i++) 
        { 
            for (j = 0; j <= n; j++) 
            { 
                if (i == 0 || j == 0) 
                    L[i, j] = 0; 
  
                else if (X[i - 1] == Y[j - 1]) 
                    L[i, j] = L[i - 1, j - 1] + 1; 
  
                else
                    L[i, j] = Math.Max(L[i - 1, j], L[i, j - 1]); 
            } 
        } 
  
        /* L[m,n] contains length of LCS for 
        X[0..n-1] and Y[0..m-1] */
        return L[m,n]; 
    } 
  
    // LCS based function to find minimum number 
    // of insersions 
    static int findMinInsertionsLCS(string str, int n) 
    { 
        // Using charArray to reverse a String 
        char[] charArray = str.ToCharArray(); 
        Array.Reverse(charArray); 
        string revString = new string(charArray); 
  
        // The output is length of string minus 
        // length of lcs of str and it reverse 
        return (n - lcs(str, revString , n, n)); 
    } 
  
    // Driver code 
    static void Main() 
    { 
        string str = "geeks"; 
        Console.WriteLine(findMinInsertionsLCS(str,str.Length)); 
    } 
} 
  
// This code is contributed by mits 

Output:

Time complexity: O(N^2)
Auxiliary Space: O(N^2)

This article is compiled by Aashish Barnwal. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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My Personal Notes

Minimum insertions to form a palindrome | DP-28

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

C++

C

Java

Python 3

C#

C++

C

Java

Python3

C#

C++

C

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