Program to find second most frequent character

Given a string, find the second most frequent character in it. Expected time complexity is O(n) where n is the length of the input string.

Examples:

Input: str = "aabababa";
Output: Second most frequent character is 'b'

Input: str = "geeksforgeeks";
Output: Second most frequent character is 'g'

Input: str = "geeksquiz";
Output: Second most frequent character is 'g'
The output can also be any other character with 
count 1 like 'z', 'i'.

Input: str = "abcd";
Output: No Second most frequent character

A simple solution is to start from the first character, count its occurrences, then second character and so on. While counting these occurrence keep track of max and second max. Time complexity of this solution is O(n²).
We can solve this problem in O(n) time using a count array with size equal to 256 (Assuming characters are stored in ASCII format). Following is the implementation of the approach.

C++

#include <bits/stdc++.h> 
using namespace std; 
#define NO_OF_CHARS 256  
  
// CPP function to find the  
// second most frequent character  
// in a given string 'str'  
char getSecondMostFreq(string str)  
{  
    // count number of occurrences of every character.  
    int count[NO_OF_CHARS] = {0}, i;  
    for (i = 0; str[i]; i++)  
        (count[str[i]])++;  
  
    // Traverse through the count[] and 
    // find second highest element.  
    int first = 0, second = 0;  
    for (i = 0; i < NO_OF_CHARS; i++)  
    {  
        /* If current element is smaller  
        than first then update both  
        first and second */
        if (count[i] > count[first])  
        {  
            second = first;  
            first = i;  
        }  
  
        /* If count[i] is in between first  
        and second then update second */
        else if (count[i] > count[second] &&  
                count[i] != count[first])  
            second = i;  
    }  
  
    return second;  
}  
  
// Driver code  
int main()  
{  
    string str = "geeksforgeeks";  
    char res = getSecondMostFreq(str);  
    if (res != '\0')  
        cout << "Second most frequent char is " << res;  
    else
        cout << "No second most frequent character";  
    return 0;  
}  
  
// This code is contributed by rathbhupendra 

C

#include <stdio.h> 
#define NO_OF_CHARS 256 
  
// C function to find the second most frequent character 
// in a given string 'str' 
char getSecondMostFreq(char *str) 
{ 
    // count number of occurrences of every character.  
    int count[NO_OF_CHARS] = {0}, i; 
    for (i=0; str[i]; i++) 
        (count[str[i]])++; 
  
    // Traverse through the count[] and find second highest element. 
    int first = 0, second = 0; 
    for (i = 0; i < NO_OF_CHARS; i++) 
    { 
        /* If current element is smaller than first then update both 
          first and second */
        if (count[i] > count[first]) 
        { 
            second = first; 
            first = i; 
        } 
  
        /* If count[i] is in between first and second then update second  */
        else if (count[i] > count[second] && 
                 count[i] != count[first]) 
            second = i; 
    } 
  
    return second; 
} 
  
// Driver program to test above function 
int main() 
{ 
  char str[] = "geeksforgeeks"; 
  char res = getSecondMostFreq(str); 
  if (res != '\0') 
     printf("Second most frequent char is %c", res); 
  else
     printf("No second most frequent character"); 
  return 0; 
} 

Java

// Java Program to find the second  
// most frequent character in a given string 
public class GFG  
{ 
    static final int NO_OF_CHARS = 256; 
      
    // finds the second most frequently occurring  
    // char 
    static char getSecondMostFreq(String str) 
    { 
        // count number of occurrences of every 
        // character.  
        int[] count = new int[NO_OF_CHARS]; 
        int i; 
        for (i=0; i< str.length(); i++) 
            (count[str.charAt(i)])++; 
       
        // Traverse through the count[] and find 
        // second highest element. 
        int first = 0, second = 0; 
        for (i = 0; i < NO_OF_CHARS; i++) 
        { 
            /* If current element is smaller than 
            first then update both first and second */
            if (count[i] > count[first]) 
            { 
                second = first; 
                first = i; 
            } 
       
            /* If count[i] is in between first and 
            second then update second  */
            else if (count[i] > count[second] && 
                     count[i] != count[first]) 
                second = i; 
        } 
       
        return (char)second; 
    } 
       
    // Driver program to test above function 
    public static void main(String args[]) 
    { 
      String str = "geeksforgeeks"; 
      char res = getSecondMostFreq(str); 
      if (res != '\0') 
         System.out.println("Second most frequent char"+ 
                                       " is " + res); 
      else
         System.out.println("No second most frequent"+ 
                                       "character"); 
    } 
} 
// This code is contributed by Sumit Ghosh 

Python 3

# Python 3 Program to find the  
# second most frequent character  
# in a given string 
  
# Function to find the second 
# most frequent character 
# in a given string 'str' 
def getSecondMostFreq(str) : 
  
    NO_OF_CHARS = 256
  
    # Initialize count list of 
    # 256 size with value 0 
    count = [0] * NO_OF_CHARS 
  
    # count number of occurrences 
    # of every character.  
    for i in range(len(str)) : 
        count[ord(str[i])] += 1
  
    first, second = 0, 0
  
    # Traverse through the count[]  
    # and find second highest element. 
    for i in range(NO_OF_CHARS) : 
  
        # If current element is smaller  
        # than first then update both 
        # first and second  
        if count[i] > count[first] : 
  
            second = first 
            first = i 
  
        # If count[i] is in between  
        # first and second 
        # then update second */ 
        elif (count[i] > count[second] and
            count[i] != count[first] ) : 
              
            second = i 
  
    # return character  
    return chr(second) 
  
# Driver code 
if __name__ == "__main__" : 
  
    str = "geeksforgeeks"
      
    # function calling  
    res = getSecondMostFreq(str) 
    if res != '\0' : 
        print("Second most frequent char is", res) 
    else : 
        print("No second most frequent character") 
  
# This code is contributed by ANKITRAI1 

C#

// C# Program to find the second most frequent 
// character in a given string 
using System; 
  
public class GFG { 
      
    static int NO_OF_CHARS = 256; 
      
    // finds the second most frequently 
    // occurring char 
    static char getSecondMostFreq(string str) 
    { 
          
        // count number of occurrences of every 
        // character.  
        int []count = new int[NO_OF_CHARS]; 
  
        for (int i = 0; i < str.Length; i++) 
            (count[str[i]])++; 
      
        // Traverse through the count[] and find 
        // second highest element. 
        int first = 0, second = 0; 
          
        for (int i = 0; i < NO_OF_CHARS; i++) 
        { 
              
            /* If current element is smaller  
            than first then update both first  
            and second */
            if (count[i] > count[first]) 
            { 
                second = first; 
                first = i; 
            } 
      
            /* If count[i] is in between first 
            and second then update second */
            else if (count[i] > count[second] && 
                       count[i] != count[first]) 
                second = i; 
        } 
      
        return (char)second; 
    } 
      
    // Driver program to test above function 
    public static void Main() 
    { 
        string str = "geeksforgeeks"; 
        char res = getSecondMostFreq(str); 
          
        if (res != '\0') 
            Console.Write("Second most frequent char"+ 
                                        " is " + res); 
        else
            Console.Write("No second most frequent"+ 
                                        "character"); 
    } 
} 
  
// This code is contributed by nitin mittal. 

PHP

<?php 
$NO_OF_CHARS=256;  
  
// PHP function to find the  
// second most frequent character  
// in a given string 'str'  
function getSecondMostFreq($str)  
{  
    global $NO_OF_CHARS; 
      
    // count number of occurrences of every character.  
    $count=array_fill(0,$NO_OF_CHARS,0);  
    for ($i = 0; $i < strlen($str); $i++)  
        $count[ord($str[$i])]++;  
  
    // Traverse through the count[] and 
    // find second highest element.  
    $first = $second = 0;  
    for ($i = 0; $i < $NO_OF_CHARS; $i++)  
    {  
        /* If current element is smaller  
        than first then update both  
        first and second */
        if ($count[$i] > $count[$first])  
        {  
            $second = $first;  
            $first = $i;  
        }  
  
        /* If count[i] is in between first  
        and second then update second */
        else if ($count[$i] > $count[$second] &&  
                $count[$i] != $count[$first])  
            $second = $i;  
    }  
  
    return chr($second);  
}  
  
    // Driver code  
    $str = "geeksforgeeks";  
    $res = getSecondMostFreq($str);  
    if (strlen($res))  
        echo "Second most frequent char is ".$res;  
    else
        echo "No second most frequent character";  
  
// This code is contributed by mits 
?> 

Output:

Second most frequent char is g

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