Minimum length of Run Length Encoding possible by removing at most K characters from a given string

Last Updated : 29 Jun, 2021

Given a string S of length N, consisting of lowercase English alphabets only, the task is to find the minimum possible length of run-length-encoding that can be generated by removing at most K characters from the string S.

Examples:

Input: S = “abbbcdcdd”, N = 9, K = 2
Output: 5
Explanation: One possible way is to delete both occurrences of ‘c’ from S.
The new string generated is “abbbddd” whose run-length-encoding is “ab3d3”.
Therefore, the length of the encoded string is 5.

Input: S = “aabbca”, N = 6, K = 3
Output: 2
Explanation: One possible way is to delete both the occurrences of ‘b’ and one occurrence of ‘c’.
The new string generated is “aaa” whose run-length-encoding is “a3”.
Therefore, the length of the encoded string is 2

Naive Approach: The simplest approach to solve the problem is to remove every combination of K characters from the string and calculate their respective run-length-encoding. Finally, print the length of the smallest run-length-encoding obtained.

Time Complexity: O(K * N!(N – K)! * K!)
Auxiliary Space: O(K)

Efficient Approach: To optimize the above approach, follow the steps below to solve the problem:

Maintain an auxiliary array dp[n][k][26][n], where dp[idx][K][last][count] denotes the minimum run-length-encoding obtained starting from index idx where, K denotes the number of deletions remaining, last denotes the last character with frequency count so far.
For every character, two possibilities exists, either to delete the character or to retain it.
Consider that the current character at index idx is deleted and calculate recursively the minimum run-length encoding obtained by passing the parameters (idx + 1, K – 1, last, count)
Now, consider that the current character at index idx is retained and calculate recursively the minimum run-length encoding for the following two cases:
If S[idx] = last: Calculate minimum run-length encoding by passing the parameters (idx + 1, K, S[idx], count + 1).
Otherwise, calculate minimum run-length encoding by passing the parameters (idx + 1, K, S[idx], 1).
Return the minimum of the above-computed values and repeat the above steps for all indices of the string.

Below is the implementation of the above approach:

C++

   // C++ Program to implement
// the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
#define maxN 20
 
int dp[maxN][maxN][27][maxN];
 
// Function which solves the desired problem
int solve(string& s, int n, int idx,
          int k, char last = 123,
          int count = 0)
{
    // idx: current index in s
    // k: Remaining number of deletions
    // last: Previous character
    // count: Number of occurrences
    // of the previous character
 
    // Base Case
    if (k < 0)
        return n + 1;
 
    // If the entire string has
    // been traversed
    if (idx == n)
        return 0;
 
    int& ans = dp[idx][k][last - 'a'][count];
 
    // If precomputed subproblem
    // occurred
    if (ans != -1)
        return ans;
 
    ans = n + 1;
 
    // Minimum run length encoding by
    // removing the current character
    ans = min(ans,
              solve(s, n, idx + 1, k - 1, last, count));
 
    // Minimum run length encoding by
    // retaining the current character
    int inc = 0;
 
    if (count == 1 || count == 9
        || count == 99)
        inc = 1;
 
    // If the current and the
    // previous characters match
    if (last == s[idx]) {
 
        ans = min(ans,
                  inc + solve(s, n, idx + 1, k, s[idx],
                              count + 1));
    }
 
    // Otherwise
    else {
 
        ans = min(ans,
                  1 + solve(s, n, idx + 1, k, s[idx], 1));
    }
 
    return ans;
}
 
// Function to return minimum run-length encoding
// for string s by removing atmost k characters
int MinRunLengthEncoding(string& s, int n, int k)
{
    memset(dp, -1, sizeof(dp));
    return solve(s, n, 0, k);
}
 
// Driver Code
int main()
{
    string S = "abbbcdcdd";
    int N = 9, K = 2;
    cout << MinRunLengthEncoding(S, N, K);
 
    return 0;
}
 
  

Java

   // Java program to implement 
// the above approach 
import java.util.*;
 
class GFG{
 
static int maxN = 20;
 
static int dp[][][][] = new int[maxN][maxN][27][maxN];
 
// Function which solves the desired problem
public static int solve(String s, int n, 
                         int idx, int k,
                       char last, int count)
{
     
    // idx: current index in s
    // k: Remaining number of deletions
    // last: Previous character
    // count: Number of occurrences
    // of the previous character
 
    // Base Case
    if (k < 0)
        return n + 1;
 
    // If the entire string has
    // been traversed
    if (idx == n)
        return 0;
 
    int ans = dp[idx][k][last - 'a'][count];
 
    // If precomputed subproblem
    // occurred
    if (ans != -1)
        return ans;
 
    ans = n + 1;
 
    // Minimum run length encoding by
    // removing the current character
    ans = Math.min(ans, solve(s, n, idx + 1, 
                              k - 1, last, 
                              count));
 
    // Minimum run length encoding by
    // retaining the current character
    int inc = 0;
 
    if (count == 1 || count == 9 || count == 99)
        inc = 1;
 
    // If the current and the
    // previous characters match
    if (last == s.charAt(idx)) 
    {
        ans = Math.min(ans, inc + solve(s, n, idx + 1,
                                        k, s.charAt(idx),
                                        count + 1));
    }
 
    // Otherwise
    else
    {
        ans = Math.min(ans, 1 + solve(s, n, idx + 1, k,
                                      s.charAt(idx), 1));
    }
    return dp[idx][k][last - 'a'][count] = ans;
}
 
// Function to return minimum run-length encoding
// for string s by removing atmost k characters
public static int MinRunLengthEncoding(String s, int n,
                                                 int k)
{
    for(int i[][][] : dp)
        for(int j[][] : i)
            for(int p[] : j)
                Arrays.fill(p, -1);
                 
    return solve(s, n, 0, k, (char)123, 0);
}
 
// Driver Code
public static void main(String args[])
{
    String S = "abbbcdcdd";
    int N = 9, K = 2;
     
    System.out.println(MinRunLengthEncoding(S, N, K));
}
}
 
// This code is contributed by hemanth gadarla
 
  

Python3

   # Python3 program to implement
# the above approach
maxN = 20
 
dp = [[[[0 for i in range(maxN)] 
           for j in range(27)] 
           for k in range(27)] 
           for l in range(maxN)]
 
# Function which solves the desired problem
def solve(s, n, idx, k, last, count):
     
    # idx: current index in s
    # k: Remaining number of deletions
    # last: Previous character
    # count: Number of occurrences
    # of the previous character
 
    # Base Case
    if (k < 0):
        return n + 1
 
    # If the entire string has
    # been traversed
    if (idx == n):
        return 0
 
    ans = dp[idx][k][ord(last) - ord('a')][count]
 
    # If precomputed subproblem
    # occurred
    if (ans != -1):
        return ans
 
    ans = n + 1
 
    # Minimum run length encoding by
    # removing the current character
    ans = min(ans, solve(s, n, idx + 1, 
                         k - 1, last, count))
 
    # Minimum run length encoding by
    # retaining the current character
    inc = 0
 
    if (count == 1 or count == 9 or
        count == 99):
        inc = 1
 
    # If the current and the
    # previous characters match
    if (last == s[idx]):
        ans = min(ans, inc + solve(s, n, idx + 1, k,
                                   s[idx], count + 1))
 
    # Otherwise
    else:
        ans = max(ans, 1 + solve(s, n, idx + 1,
                                 k, s[idx], 1))
                                  
    dp[idx][k][ord(last) - ord('a')][count] = ans
    #print(ans)
     
    return dp[idx][k][ord(last) - ord('a')][count]
 
# Function to return minimum run-length encoding
# for string s by removing atmost k characters
def MinRunLengthEncoding(s, n, k):
     
    for i in range(maxN):
        for j in range(27):
            for k in range(27):
                for l in range(maxN):
                    dp[i][j][k][l] = -1
                     
    return solve(s, n, 0, k, chr(123), 0) - 1
 
# Driver Code
if __name__ == '__main__':
     
    S = "abbbcdcdd"
    N = 9
    K = 2
 
    print(MinRunLengthEncoding(S, N, K))
 
# This code is contributed by gauravrajput1
 
  

C#

   // C# program to implement 
// the above approach 
using System;
class GFG{
 
static int maxN = 20;
 
static int [,,,]dp = 
       new int[maxN, maxN,
               27, maxN];
 
// Function which solves 
// the desired problem
public static int solve(String s, int n, 
                        int idx, int k,
                        char last, int count)
{    
  // idx: current index in s
  // k: Remaining number of deletions
  // last: Previous character
  // count: Number of occurrences
  // of the previous character
 
  // Base Case
  if (k < 0)
    return n + 1;
 
  // If the entire string 
  // has been traversed
  if (idx == n)
    return 0;
 
  int ans = dp[idx, k, last - 
               'a', count];
 
  // If precomputed subproblem
  // occurred
  if (ans != -1)
    return ans;
 
  ans = n + 1;
 
  // Minimum run length encoding by
  // removing the current character
  ans = Math.Min(ans, 
                 solve(s, n, idx + 1, 
                       k - 1, last,
                       count));
 
  // Minimum run length encoding by
  // retaining the current character
  int inc = 0;
 
  if (count == 1 || count == 9 || 
      count == 99)
    inc = 1;
 
  // If the current and the
  // previous characters match
  if (last == s[idx]) 
  {
    ans = Math.Min(ans, inc + 
                   solve(s, n, idx + 1,
                         k, s[idx],
                         count + 1));
  }
 
  // Otherwise
  else
  {
    ans = Math.Min(ans, 1 + 
                   solve(s, n, idx + 1, 
                         k, s[idx], 1));
  }
  return dp[idx, k, last - 
            'a', count] = ans;
}
 
// Function to return minimum 
// run-length encoding for string 
// s by removing atmost k characters
public static int MinRunLengthEncoding(String s, 
                                       int n, int k)
{
  for (int i = 0; i < maxN; i++)
    for (int j = 0; j < maxN; j++)
      for (int p = 0; p < 27; p++)
        for (int l = 0; l < maxN; l++)
          dp[i, j, p, l] = -1;
 
  return solve(s, n, 0, 
               k, (char)123, 0);
}
 
// Driver Code
public static void Main(String []args)
{
  String S = "abbbcdcdd";
  int N = 9, K = 2;
  Console.WriteLine(
          MinRunLengthEncoding(S,
                               N, K));
}
}
 
// This code is contributed by 29AjayKumar
 
  

Javascript

   <script>
 
    // JavaScript program to implement the above approach
     
    let maxN = 20;
  
    let dp = new Array(maxN);
 
    // Function which solves the desired problem
    function solve(s, n, idx, k, last, count)
    {
 
        // idx: current index in s
        // k: Remaining number of deletions
        // last: Previous character
        // count: Number of occurrences
        // of the previous character
 
        // Base Case
        if (k < 0)
            return n + 1;
 
        // If the entire string has
        // been traversed
        if (idx == n)
            return 0;
 
        let ans = dp[idx][k][last - 'a'.charCodeAt()][count];
 
        // If precomputed subproblem
        // occurred
        if (ans != -1)
            return ans;
 
        ans = n + 1;
 
        // Minimum run length encoding by
        // removing the current character
        ans = Math.min(ans, solve(s, n, idx + 1,
                                  k - 1, last,
                                  count));
 
        // Minimum run length encoding by
        // retaining the current character
        let inc = 0;
 
        if (count == 1 || count == 9 || count == 99)
            inc = 1;
 
        // If the current and the
        // previous characters match
        if (last == s[idx].charCodeAt())
        {
            ans = Math.min(ans, inc + solve(s, n, idx + 1,
                                            k, s[idx].charCodeAt(),
                                            count + 1));
        }
 
        // Otherwise
        else
        {
            ans = Math.min(ans, 1 + solve(s, n, idx + 1, k,
                                          s[idx].charCodeAt(), 1));
        }
        dp[idx][k][last - 'a'.charCodeAt()][count] = ans;
        return dp[idx][k][last - 'a'.charCodeAt()][count];
    }
 
    // Function to return minimum run-length encoding
    // for string s by removing atmost k characters
    function MinRunLengthEncoding(s, n, k)
    {
        for(let i = 0; i < maxN; i++)
        {
            dp[i] = new Array(maxN);
            for(let j = 0; j < maxN; j++)
            {
                dp[i][j] = new Array(27);
                for(let k = 0; k < 27; k++)
                {
                    dp[i][j][k] = new Array(maxN);
                    for(let l = 0; l < maxN; l++)
                    {
                        dp[i][j][k][l] = -1;
                    }
                }
            }
        }
 
        return solve(s, n, 0, k, 123, 0);
    }
     
    let S = "abbbcdcdd";
    let N = 9, K = 2;
      
    document.write(MinRunLengthEncoding(S, N, K));
     
</script>
 
  

Output:

Time Complexity: O(26 * N² * K)
Auxiliary Space: O(26 * N² * K)

Minimum operations required to convert all characters of a String to a given Character

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Minimum length of Run Length Encoding possible by removing at most K characters from a given string

C++

Java

Python3

C#

Javascript

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