Concatenate strings in any order to get Maximum Number of “AB”

Given an array of strings of length N, it is allowed to concatenate them in any order. Find the maximum possible number of occurrences of ‘AB’ in the resulting string.

Examples:

Input : N = 4, arr={ “BCA”, “BGGGA”, “JKA”, “BALB” }
Output : 3
Concatenate them in the order JKA + BGGA + BCA + BALB and it will become JKABGGABCABALB and it has3 occurrences of ‘AB’.

Input : N = 3, arr={ “ABCA”, “BOOK”, “BAND” }
Output : 3

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach:
Calculate the number of ABs within each string beforehand. Concentrate on the change of the number of ABs that extend over two strings when the strings are rearranged. The only characters that matter in each string are its first and last characters.

The strings that can contribute to the answer are:

1. A string that begins with B and ends with A.
2. A string that begins with B but does not end with A.
3. A string that does not begin with B but ends with A.

Let c1, c2, and c3 be the number of strings of category 1, 2 and 3, respectively.

• If c1 = 0 then the answer is min(c2, c3) as we can take both and concatenate as long as both are available.
• If c1 > 0 and c2 + c3 = 0 then answer is c1 – 1 as we concatenate then in serial order one by one.
• If c1 > 0 and c2 + c3 > 0 and take min(c2, c3) = p, first concatenate category 1 string one by one and the extra c1 – 1 ‘AB’ and then if both category 2 and 3 available then add category 3 at beginning of present resulting string and add category 2 at the end of present resulting string.
• There are c1 – 1 + 2 = c1 + 1 extra ‘AB’ and now c2 and c3 decreased by one and p become p – 1 and now take both
category 2 and 3 and add them as long as both are available and now we get total c1 + 1 + (p – 1) = c1 + p extra ‘AB’ that means c1 + min(c2, c3) .

Below is the implementation of the above approach:

C++

 // C++ implementation of above approach #include using namespace std;    // Function to find maximum number of ABs int maxCountAB(string s[], int n) {     // variable A, B, AB for count strings that     // end with 'A' but not end with 'B', 'B' but     // does not end with 'A' and 'B' and ends     // with 'A' respectively.     int A = 0, B = 0, BA = 0, ans = 0;        for (int i = 0; i < n; i++) {         string S = s[i];         int L = S.size();         for (int j = 0; j < L - 1; j++) {                // 'AB' is already present in string             // before concatenate them             if (S.at(j) == 'A' &&                             S.at(j + 1) == 'B') {                 ans++;             }         }            // count of strings that begins         // with 'B' and ends with 'A         if (S.at(0) == 'B' && S.at(L - 1) == 'A')             BA++;            // count of strings that begins         // with 'B' but does not end with 'A'         else if (S.at(0) == 'B')             B++;            // count of strings that ends with         // 'A' but not end with 'B'         else if (S.at(L - 1) == 'A')             A++;     }        // updating the value of ans and     // add extra count of 'AB'     if (BA == 0)         ans += min(B, A);     else if (A + B == 0)         ans += BA - 1;     else         ans += BA + min(B, A);        return ans; }    // Driver Code int main() {        string s[] = { "ABCA", "BOOK", "BAND" };        int n = sizeof(s) / sizeof(s[0]);        cout << maxCountAB(s, n);        return 0; }

Java

 // Java implementation of above approach import java.util.*;    class GFG {        // Function to find maximum number of ABs static int maxCountAB(String s[], int n) {     // variable A, B, AB for count strings that     // end with 'A' but not end with 'B', 'B' but     // does not end with 'A' and 'B' and ends     // with 'A' respectively.     int A = 0, B = 0, BA = 0, ans = 0;        for (int i = 0; i < n; i++)     {         String S = s[i];         int L = S.length();         for (int j = 0; j < L - 1; j++)          {                // 'AB' is already present in string             // before concatenate them             if (S.charAt(j) == 'A' &&                          S.charAt(j + 1) == 'B')              {                 ans++;             }         }            // count of strings that begins         // with 'B' and ends with 'A         if (S.charAt(0) == 'B' && S.charAt(L - 1) == 'A')             BA++;            // count of strings that begins         // with 'B' but does not end with 'A'         else if (S.charAt(0) == 'B')             B++;            // count of strings that ends with         // 'A' but not end with 'B'         else if (S.charAt(L - 1) == 'A')             A++;     }        // updating the value of ans and     // add extra count of 'AB'     if (BA == 0)         ans += Math.min(B, A);     else if (A + B == 0)         ans += BA - 1;     else         ans += BA + Math.min(B, A);        return ans; }    // Driver Code public static void main(String[] args)  {     String s[] = { "ABCA", "BOOK", "BAND" };        int n = s.length;        System.out.println(maxCountAB(s, n)); } }    // This code is contributed by 29AjayKumar

Python3

 # Python 3 implementation of above approach    # Function to find maximum number of ABs def maxCountAB(s,n):     # variable A, B, AB for count strings that     # end with 'A' but not end with 'B', 'B' but     # does not end with 'A' and 'B' and ends     # with 'A' respectively.     A = 0     B = 0     BA = 0     ans = 0        for i in range(n):         S = s[i]         L = len(S)         for j in range(L-1):             # 'AB' is already present in string             # before concatenate them             if (S[j] == 'A' and S[j + 1] == 'B'):                 ans += 1            # count of strings that begins         # with 'B' and ends with 'A         if (S[0] == 'B' and S[L - 1] == 'A'):             BA += 1            # count of strings that begins         # with 'B' but does not end with 'A'         elif (S[0] == 'B'):             B += 1            # count of strings that ends with         # 'A' but not end with 'B'         elif (S[L - 1] == 'A'):             A += 1        # updating the value of ans and     # add extra count of 'AB'     if (BA == 0):         ans += min(B, A)     elif (A + B == 0):         ans += BA - 1     else:         ans += BA + min(B, A)     return ans    # Driver Code if __name__ == '__main__':     s = ["ABCA", "BOOK", "BAND"]        n = len(s)        print(maxCountAB(s, n))    # This code is contributed by # Surendra_Gangwar

C#

 // C# implementation of above approach  using System;    class GFG  {                 // Function to find maximum number of ABs      static int maxCountAB(string []s, int n)      {          // variable A, B, AB for count strings that          // end with 'A' but not end with 'B', 'B' but          // does not end with 'A' and 'B' and ends          // with 'A' respectively.          int A = 0, B = 0, BA = 0, ans = 0;                 for (int i = 0; i < n; i++)          {              string S = s[i];              int L = S.Length;              for (int j = 0; j < L - 1; j++)              {                         // 'AB' is already present in string                  // before concatenate them                  if (S[j] == 'A' &&                              S[j + 1] == 'B')                  {                      ans++;                  }              }                     // count of strings that begins              // with 'B' and ends with 'A              if (S[0] == 'B' && S[L - 1] == 'A')                  BA++;                     // count of strings that begins              // with 'B' but does not end with 'A'              else if (S[0] == 'B')                  B++;                     // count of strings that ends with              // 'A' but not end with 'B'              else if (S[L - 1] == 'A')                  A++;          }                 // updating the value of ans and          // add extra count of 'AB'          if (BA == 0)              ans += Math.Min(B, A);                         else if (A + B == 0)              ans += BA - 1;          else             ans += BA + Math.Min(B, A);                 return ans;      }             // Driver Code      public static void Main()      {          string []s = { "ABCA", "BOOK", "BAND" };                 int n = s.Length;                 Console.WriteLine(maxCountAB(s, n));      }  }     // This code is contributed by AnkitRai01

Output:

2

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