Most frequent character in a string after replacing all occurrences of X in a Binary String

• Last Updated : 03 Jun, 2021

Given a string S of length N consisting of 1, 0, and X, the task is to print the character (‘1’ or ‘0’) with the maximum frequency after replacing every occurrence of X as per the following conditions:

• If the character present adjacently to the left of X is 1, replace X with 1.
• If the character present adjacently to the right of X is 0, replace X with 0.
• If both the above conditions are satisfied, X remains unchanged.

Note: If the frequency of 1 and 0 is the same after replacements, then print X.

Examples:

Input: S = “XX10XX10XXX1XX”
Output: 1
Explanation:
Operation 1: S = “X11001100X1XX”
Operation 2: S = “111001100X1XX”
No further replacements are possible.
Hence, the frequencies of ‘1’ and ‘0’ are 6 and 4 respectively.

Input: S = “0XXX1”
Output: X
Explanation:
Operation 1: S = “00X11”
No further replacements are possible.
Hence, the frequencies of both ‘1’ and ‘0’ are 2.

Approach: The given problem can be solved based on the following observations:

• All the ‘X’s lying between ‘1’ and ‘0’ (e.g. 1XXX0) is of no significance because neither of ‘1’ and ‘0’ can convert it.
• All the ‘X’s lying between ‘0’ and ‘1’ (e.g. 0XXX1) is also of no significance because it will contribute equally to both 1 and 0. Consider any substring of the form “0X….X1”, then after changing the first occurrence of X from the start and the end of the string, the actual frequency of 0 and 1 in the substring remains unchanged.

From the above observations it can be concluded that the result depends upon the following conditions:

• The count of ‘1’ and ‘0’ in the original string.
• The frequency of X that are present between two consecutive 0s or two consecutive 1s, i.e. “0XXX0” and “1XXXX1” respectively.
• The number of continuous ‘X’ which are present at the starting of string and has a right end ‘1’, i.e. “XXXX1…..”.
• The number of continuous ‘X’s which are present at end of the string and has a left end ‘0’ i.e., …..0XXX.

Hence, count the number of 1s and 0s as per the above conditions and print the resultant count.

Below is the implementation of the above approach:

C++

 // C++ program for the above approach#include using namespace std; // Function to find the most frequent// character after replacing X with// either '0' or '1' according as per// the given conditionsvoid maxOccuringCharacter(string s){     // Store the count of 0s and    // 1s in the string S    int count0 = 0, count1 = 0;     // Count the frequency of    // 0 and 1    for (int i = 0; i < s.length(); i++) {         // If the character is 1        if (s[i] == '1') {            count1++;        }         // If the character is 0        else if (s[i] == '0') {            count0++;        }    }     // Stores first occurence of 1    int prev = -1;    for (int i = 0; i < s.length(); i++) {         if (s[i] == '1') {            prev = i;            break;        }    }     // Traverse the string to count    // the number of X between two    // consecutive 1s    for (int i = prev + 1; i < s.length(); i++) {         // If the current character        // is not X        if (s[i] != 'X') {             // If the current character            // is 1, add the number of            // Xs to count1 and set            // prev to i            if (s[i] == '1') {                count1 += i - prev - 1;                prev = i;            }             // Otherwise            else {                 // Find next occurence                // of 1 in the string                bool flag = true;                 for (int j = i + 1; j < s.length(); j++) {                    if (s[j] == '1') {                        flag = false;                        prev = j;                        break;                    }                }                 // If it is found,                // set i to prev                if (!flag) {                    i = prev;                }                 // Otherwise, break                // out of the loop                else {                    i = s.length();                }            }        }    }     // Store the first occurence of 0    prev = -1;    for (int i = 0; i < s.length(); i++) {         if (s[i] == '0') {            prev = i;            break;        }    }     // Repeat the same procedure to    // count the number of X between    // two consecutive 0s    for (int i = prev + 1; i < s.length(); i++) {         // If the current character is not X        if (s[i] != 'X') {             // If the current character is 0            if (s[i] == '0') {                 // Add the count of Xs to count0                count0 += i - prev - 1;                 // Set prev to i                prev = i;            }             // Otherwise            else {                 // Find the next occurence                // of 0 in the string                bool flag = true;                 for (int j = i + 1; j < s.length(); j++) {                     if (s[j] == '0') {                        prev = j;                        flag = false;                        break;                    }                }                 // If it is found,                // set i to prev                if (!flag) {                    i = prev;                }                 // Otherwise, break out                // of the loop                else {                    i = s.length();                }            }        }    }     // Count number of X present in    // the starting of the string    // as XXXX1...    if (s == 'X') {         // Store the count of X        int count = 0;        int i = 0;        while (s[i] == 'X') {            count++;            i++;        }         // Increment count1 by        // count if the condition        // is satisfied        if (s[i] == '1') {            count1 += count;        }    }     // Count the number of X    // present in the ending of    // the string as ...XXXX0    if (s[(s.length() - 1)] == 'X') {         // Store the count of X        int count = 0;        int i = s.length() - 1;        while (s[i] == 'X') {            count++;            i--;        }         // Increment count0 by        // count if the condition        // is satisfied        if (s[i] == '0') {            count0 += count;        }    }     // If count of 1 is equal to    // count of 0, print X    if (count0 == count1) {        cout << "X" << endl;    }     // Otherwise, if count of 1    // is greater than count of 0    else if (count0 > count1) {        cout << 0 << endl;    }     // Otherwise, print 0    else        cout << 1 << endl;} // Driver Codeint main(){    string S = "XX10XX10XXX1XX";    maxOccuringCharacter(S);} // This code is contributed by SURENDAR_GANGWAR.

Java

 // Java program for the above approachimport java.io.*; class GFG {     // Function to find the most frequent    // character after replacing X with    // either '0' or '1' according as per    // the given conditions    public static void    maxOccuringCharacter(String s)    {        // Store the count of 0s and        // 1s in the string S        int count0 = 0, count1 = 0;         // Count the frequency of        // 0 and 1        for (int i = 0;             i < s.length(); i++) {             // If the character is 1            if (s.charAt(i) == '1') {                count1++;            }             // If the character is 0            else if (s.charAt(i) == '0') {                count0++;            }        }         // Stores first occurence of 1        int prev = -1;         for (int i = 0;             i < s.length(); i++) {             if (s.charAt(i) == '1') {                prev = i;                break;            }        }         // Traverse the string to count        // the number of X between two        // consecutive 1s        for (int i = prev + 1;             i < s.length(); i++) {             // If the current character            // is not X            if (s.charAt(i) != 'X') {                 // If the current character                // is 1, add the number of                // Xs to count1 and set                // prev to i                if (s.charAt(i) == '1') {                    count1 += i - prev - 1;                    prev = i;                }                 // Otherwise                else {                     // Find next occurence                    // of 1 in the string                    boolean flag = true;                     for (int j = i + 1;                         j < s.length();                         j++) {                        if (s.charAt(j) == '1') {                            flag = false;                            prev = j;                            break;                        }                    }                     // If it is found,                    // set i to prev                    if (!flag) {                        i = prev;                    }                     // Otherwise, break                    // out of the loop                    else {                        i = s.length();                    }                }            }        }         // Store the first occurence of 0        prev = -1;        for (int i = 0; i < s.length(); i++) {             if (s.charAt(i) == '0') {                prev = i;                break;            }        }         // Repeat the same procedure to        // count the number of X between        // two consecutive 0s        for (int i = prev + 1;             i < s.length(); i++) {             // If the current character is not X            if (s.charAt(i) != 'X') {                 // If the current character is 0                if (s.charAt(i) == '0') {                     // Add the count of Xs to count0                    count0 += i - prev - 1;                     // Set prev to i                    prev = i;                }                 // Otherwise                else {                     // Find the next occurence                    // of 0 in the string                    boolean flag = true;                     for (int j = i + 1;                         j < s.length(); j++) {                         if (s.charAt(j) == '0') {                            prev = j;                            flag = false;                            break;                        }                    }                     // If it is found,                    // set i to prev                    if (!flag) {                        i = prev;                    }                     // Otherwise, break out                    // of the loop                    else {                        i = s.length();                    }                }            }        }         // Count number of X present in        // the starting of the string        // as XXXX1...        if (s.charAt(0) == 'X') {             // Store the count of X            int count = 0;            int i = 0;            while (s.charAt(i) == 'X') {                count++;                i++;            }             // Increment count1 by            // count if the condition            // is satisfied            if (s.charAt(i) == '1') {                count1 += count;            }        }         // Count the number of X        // present in the ending of        // the string as ...XXXX0        if (s.charAt(s.length() - 1)            == 'X') {             // Store the count of X            int count = 0;            int i = s.length() - 1;            while (s.charAt(i) == 'X') {                count++;                i--;            }             // Increment count0 by            // count if the condition            // is satisfied            if (s.charAt(i) == '0') {                count0 += count;            }        }         // If count of 1 is equal to        // count of 0, print X        if (count0 == count1) {            System.out.println("X");        }         // Otherwise, if count of 1        // is greater than count of 0        else if (count0 > count1) {            System.out.println(0);        }         // Otherwise, print 0        else            System.out.println(1);    }     // Driver Code    public static void main(String[] args)    {        String S = "XX10XX10XXX1XX";        maxOccuringCharacter(S);    }}

Python3

 # Python program for the above approach # Function to find the most frequent# character after replacing X with# either '0' or '1' according as per# the given conditionsdef maxOccuringCharacter(s):     # Store the count of 0s and  # 1s in the S  count0 = 0  count1 = 0   # Count the frequency of  # 0 and 1  for i in range(len(s)):     # If the character is 1    if (s[i] == '1') :      count1 += 1         # If the character is 0    elif (s[i] == '0') :      count0 += 1       # Stores first occurence of 1  prev = -1  for i in range(len(s)):    if (s[i] == '1') :      prev = i      break       # Traverse the to count  # the number of X between two  # consecutive 1s  for i in range(prev + 1, len(s)):     # If the current character    # is not X    if (s[i] != 'X') :       # If the current character      # is 1, add the number of      # Xs to count1 and set      # prev to i      if (s[i] == '1') :        count1 += i - prev - 1        prev = i             # Otherwise      else :         # Find next occurence        # of 1 in the string        flag = True        for j in range(i+1, len(s)):          if (s[j] == '1') :            flag = False            prev = j            break                   # If it is found,        # set i to prev        if (flag == False) :          i = prev                 # Otherwise, break        # out of the loop        else :          i = len(s)           # Store the first occurence of 0  prev = -1  for i in range(0, len(s)):     if (s[i] == '0') :      prev = i      break       # Repeat the same procedure to  # count the number of X between  # two consecutive 0s  for i in range(prev + 1, len(s)):     # If the current character is not X    if (s[i] != 'X') :       # If the current character is 0      if (s[i] == '0') :         # Add the count of Xs to count0        count0 += i - prev - 1         # Set prev to i        prev = i             # Otherwise      else :         # Find the next occurence        # of 0 in the string        flag = True         for j in range(i + 1, len(s)):          if (s[j] == '0') :            prev = j            flag = False            break                  # If it is found,        # set i to prev        if (flag == False) :          i = prev                 # Otherwise, break out        # of the loop        else :          i = len(s)          # Count number of X present in  # the starting of the string  # as XXXX1...  if (s == 'X') :     # Store the count of X    count = 0    i = 0    while (s[i] == 'X') :      count += 1      i += 1         # Increment count1 by    # count if the condition    # is satisfied    if (s[i] == '1') :      count1 += count      # Count the number of X  # present in the ending of  # the as ...XXXX0  if (s[(len(s) - 1)]      == 'X') :     # Store the count of X    count = 0    i = len(s) - 1    while (s[i] == 'X') :      count += 1      i -= 1         # Increment count0 by    # count if the condition    # is satisfied    if (s[i] == '0') :      count0 += count       # If count of 1 is equal to  # count of 0, prX  if (count0 == count1) :    print("X")     # Otherwise, if count of 1  # is greater than count of 0  elif (count0 > count1) :    print( 0 )     # Otherwise, pr0  else:    print(1) # Driver Code S = "XX10XX10XXX1XX"maxOccuringCharacter(S) # This code is contributed by sanjoy_62.

C#

 // C# program for the above approachusing System;public class GFG{   // Function to find the most frequent  // character after replacing X with  // either '0' or '1' according as per  // the given conditions  public static void maxOccuringCharacter(string s)  {     // Store the count of 0s and    // 1s in the string S    int count0 = 0, count1 = 0;     // Count the frequency of    // 0 and 1    for (int i = 0;         i < s.Length; i++) {       // If the character is 1      if (s[i] == '1') {        count1++;      }       // If the character is 0      else if (s[i] == '0') {        count0++;      }    }     // Stores first occurence of 1    int prev = -1;     for (int i = 0;         i < s.Length; i++) {       if (s[i] == '1') {        prev = i;        break;      }    }     // Traverse the string to count    // the number of X between two    // consecutive 1s    for (int i = prev + 1;         i < s.Length; i++) {       // If the current character      // is not X      if (s[i] != 'X') {         // If the current character        // is 1, add the number of        // Xs to count1 and set        // prev to i        if (s[i] == '1') {          count1 += i - prev - 1;          prev = i;        }         // Otherwise        else {           // Find next occurence          // of 1 in the string          bool flag = true;           for (int j = i + 1;               j < s.Length;               j++) {            if (s[j] == '1') {              flag = false;              prev = j;              break;            }          }           // If it is found,          // set i to prev          if (!flag) {            i = prev;          }           // Otherwise, break          // out of the loop          else {            i = s.Length;          }        }      }    }     // Store the first occurence of 0    prev = -1;    for (int i = 0; i < s.Length; i++) {       if (s[i] == '0') {        prev = i;        break;      }    }     // Repeat the same procedure to    // count the number of X between    // two consecutive 0s    for (int i = prev + 1;         i < s.Length; i++) {       // If the current character is not X      if (s[i] != 'X') {         // If the current character is 0        if (s[i] == '0') {           // Add the count of Xs to count0          count0 += i - prev - 1;           // Set prev to i          prev = i;        }         // Otherwise        else {           // Find the next occurence          // of 0 in the string          bool flag = true;           for (int j = i + 1;               j < s.Length; j++) {             if (s[j] == '0') {              prev = j;              flag = false;              break;            }          }           // If it is found,          // set i to prev          if (!flag) {            i = prev;          }           // Otherwise, break out          // of the loop          else {            i = s.Length;          }        }      }    }     // Count number of X present in    // the starting of the string    // as XXXX1...    if (s == 'X') {       // Store the count of X      int count = 0;      int i = 0;      while (s[i] == 'X') {        count++;        i++;      }       // Increment count1 by      // count if the condition      // is satisfied      if (s[i] == '1') {        count1 += count;      }    }     // Count the number of X    // present in the ending of    // the string as ...XXXX0    if (s[s.Length - 1]        == 'X') {       // Store the count of X      int count = 0;      int i = s.Length - 1;      while (s[i] == 'X') {        count++;        i--;      }       // Increment count0 by      // count if the condition      // is satisfied      if (s[i] == '0') {        count0 += count;      }    }     // If count of 1 is equal to    // count of 0, print X    if (count0 == count1) {      Console.WriteLine("X");    }     // Otherwise, if count of 1    // is greater than count of 0    else if (count0 > count1) {      Console.WriteLine(0);    }     // Otherwise, print 0    else      Console.WriteLine(1);  }   // Driver Code  public static void Main(string[] args)  {    string S = "XX10XX10XXX1XX";    maxOccuringCharacter(S);  }} // This code is contributed by AnkThon

Javascript


Output:
1

Time Complexity: O(N)
Auxiliary Space: O(1)

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