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 occurrences 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 a size equal to 256 (Assuming characters are stored in ASCII format). Following is the implementation of the approach. 
Implementation:

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
?>

Javascript

<script>

    // JavaScript Program to find
    // the second most frequent
    // character in a given string

    let NO_OF_CHARS = 256;

    // finds the second most frequently
    // occurring char
    function getSecondMostFreq(str)
    {

        // count number of occurrences of every
        // character.
        let count = new Array(NO_OF_CHARS);
        count.fill(0);

        for (let i = 0; i < str.length; i++)
            (count[str[i].charCodeAt()])++;

        // Traverse through the count[] and find
        // second highest element.
        let first = 0, second = 0;

        for (let 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 String.fromCharCode(second);
    }

    let str = "geeksforgeeks";
    let res = getSecondMostFreq(str);

    if (res != '\0')
      document.write("Second most frequent char"+
                    " is " + res);
    else
      document.write("No second most frequent"+
                    "character");

</script>

Output

Second most frequent char is g

Time Complexity: O(N), as we are using a loop for traversing the string.
Auxiliary Space: O(256), as we are using extra space for count array.