查找第二频繁字符的程序

给定一个字符串，找出其中第二频繁出现的字符。预期时间复杂度为 O(n)，其中 n 是输入字符串的长度。
例子：

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

一个简单的解决方案是从第一个字符开始，计算其出现次数，然后是第二个字符，依此类推。在计算这些事件的同时，跟踪最大值和第二个最大值。该解决方案的时间复杂度为 O(n ² )。
我们可以使用大小等于 256 的计数数组在 O(n) 时间内解决这个问题（假设字符以 ASCII 格式存储）。以下是该方法的实现。

C++

#include 
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 
#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

 $count[$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

 
    // 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");
     

输出：

Second most frequent char is g