Index of character depending on frequency count in string

Last Updated : 29 Dec, 2022

Given a string str containing only lowercase characters, the task is to answer Q queries of the following type:

1 C X: Find the largest i such that str[0...i] has exactly X occurrence of the character C.
2 C X: Find the smallest i such that str[0...i] has exactly X occurrence of the character C.

Example:

Input: str = "geeksforgeeks", query[] = {{1, 'e', 2}, {2, 'k', 2}}
Output:
8
11
Query 1: "geeksforg" is the largest substring starting at str[0] with 'e' appearing exactly twice and the index of the last character is 8.
Query 2: "geeksforgeek" is the smallest substring starting at str[0] with 'k' appearing exactly twice and the index of the last character is 11.

Input: str = "abcdabcd", query[] = {{1, 'a', 1}, {2, 'a', 2}}
Output:
3
4

Approach: Create two 2-dimensional arrays L[][] and F[][] such that L[i][j] stores the largest i such that the i^th character appears exactly j^th times in str[0...i] and F[i][j] stores the smallest i such that the i^th character appears exactly j^th times in str[0...i]. In order to do so, traverse the whole string and maintain a frequency array so that for each iteration/character, its count is updated and then start another loop from 0 to 26 (each letter a-z). In the inner loop, if the iterator is equal to character value then update L[][] and F[][] array with the current index position using outer loop iterator otherwise just increment the L[][] array value for other characters by 1 as only index has been incremented and the character has not occurred. Now, type 1 query can be answered as L[given character][Frequency count] and type 2 query as F[given character][Frequency count].

Below is the implementation of the above approach.

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

const int MAX = 26;

// Function to perform the queries
void performQueries(string str, int q, int type[],
                    char ch[], int freq[])
{

    int n = str.length();

    // L[i][j] stores the largest i
    // such that ith character appears
    // exactly jth times in str[0...i]
    int L[MAX][n];

    // F[i][j] stores the smallest i
    // such that ith character appears
    // exactly jth times in str[0...i]
    int F[MAX][n];

    // To store the frequency of each
    // of the character of str
    int cnt[MAX] = { 0 };
    for (int i = 0; i < n; i++) {

        // Current character of str
        int k = str[i] - 'a';

        // Update its frequency
        cnt[k]++;

        // For every lowercase character
        // of the English alphabet
        for (int j = 0; j < MAX; j++) {

            // If it is equal to the character
            // under consideration then update
            // L[][] and R[][] as it is cnt[j]th
            // occurrence of character k
            if (k == j) {
                L[j][cnt[j]] = i;
                F[j][cnt[j]] = i;
            }

            // Only update L[][] as k has not
            // been occurred so only index
            // has to be incremented
            else
                L[j][cnt[j]] = L[j][cnt[j]] + 1;
        }
    }

    // Perform the queries
    for (int i = 0; i < q; i++) {

        // Type 1 query
        if (type[i] == 1) {
            cout << L[ch[i] - 'a'][freq[i]];
        }

        // Type 2 query
        else {
            cout << F[ch[i] - 'a'][freq[i]];
        }

        cout << "\n";
    }
}

// Driver code
int main()
{
    string str = "geeksforgeeks";

    // Queries
    int type[] = { 1, 2 };
    char ch[] = { 'e', 'k' };
    int freq[] = { 2, 2 };
    int q = sizeof(type) / sizeof(int);

    // Perform the queries
    performQueries(str, q, type, ch, freq);

    return 0;
}

Java

// Java implementation of the approach
class GFG
{
static int MAX = 26;

// Function to perform the queries
static void performQueries(String str, int q, int type[],
                                   char ch[], int freq[])
{
    int n = str.length();

    // L[i][j] stores the largest i
    // such that ith character appears
    // exactly jth times in str[0...i]
    int [][]L = new int[MAX][n];

    // F[i][j] stores the smallest i
    // such that ith character appears
    // exactly jth times in str[0...i]
    int [][]F = new int[MAX][n];

    // To store the frequency of each
    // of the character of str
    int []cnt = new int[MAX];
    for (int i = 0; i < n; i++) 
    {

        // Current character of str
        int k = str.charAt(i) - 'a';

        // Update its frequency
        cnt[k]++;

        // For every lowercase character
        // of the English alphabet
        for (int j = 0; j < MAX; j++) 
        {

            // If it is equal to the character
            // under consideration then update
            // L[][] and R[][] as it is cnt[j]th
            // occurrence of character k
            if (k == j) 
            {
                L[j][cnt[j]] = i;
                F[j][cnt[j]] = i;
            }

            // Only update L[][] as k has not
            // been occurred so only index
            // has to be incremented
            else
                L[j][cnt[j]] = L[j][cnt[j]] + 1;
        }
    }

    // Perform the queries
    for (int i = 0; i < q; i++) 
    {

        // Type 1 query
        if (type[i] == 1)
        {
            System.out.print(L[ch[i] - 'a'][freq[i]]);
        }

        // Type 2 query
        else
        {
            System.out.print(F[ch[i] - 'a'][freq[i]]);
        }
        System.out.print("\n");
    }
}

// Driver code
public static void main(String []args) 
{
    String str = "geeksforgeeks";

    // Queries
    int type[] = { 1, 2 };
    char ch[] = { 'e', 'k' };
    int freq[] = { 2, 2 };
    int q = type.length;

    // Perform the queries
    performQueries(str, q, type, ch, freq);
}
}

// This code is contributed by Rajput-Ji

Python3

# Python3 implementation of the approach 
import numpy as np

MAX = 26; 

# Function to perform the queries 
def performQueries(string , q, type_arr, ch, freq) :

    n = len(string); 

    # L[i][j] stores the largest i 
    # such that ith character appears 
    # exactly jth times in str[0...i] 
    L = np.zeros((MAX, n)); 

    # F[i][j] stores the smallest i 
    # such that ith character appears 
    # exactly jth times in str[0...i] 
    F = np.zeros((MAX, n)); 

    # To store the frequency of each 
    # of the character of str 
    cnt = [ 0 ] * MAX; 
    for i in range(n) :

        # Current character of str 
        k = ord(string[i]) - ord('a'); 

        # Update its frequency 
        cnt[k] += 1; 

        # For every lowercase character 
        # of the English alphabet 
        for j in range(MAX) :

            # If it is equal to the character 
            # under consideration then update 
            # L[][] and R[][] as it is cnt[j]th 
            # occurrence of character k 
            if (k == j) :
                L[j][cnt[j]] = i; 
                F[j][cnt[j]] = i; 

            # Only update L[][] as k has not 
            # been occurred so only index 
            # has to be incremented 
            else :
                L[j][cnt[j]] = L[j][cnt[j]] + 1; 

    # Perform the queries 
    for i in range(q) :

        # Type 1 query 
        if (type_arr[i] == 1) :
            print(L[ord(ch[i]) - 
                    ord('a')][freq[i]], end = ""); 

        # Type 2 query 
        else :
            print(F[ord(ch[i]) - 
                    ord('a')][freq[i]], end = "");
            
        print()
        
# Driver code 
if __name__ == "__main__" : 

    string = "geeksforgeeks"; 

    # Queries 
    type_arr = [ 1, 2 ]; 
    ch = [ 'e', 'k' ]; 
    freq = [ 2, 2 ]; 
    q = len(type_arr); 

    # Perform the queries 
    performQueries(string, q, type_arr, ch, freq); 

# This code is contributed by AnkitRai01

// C# implementation of the approach
using System;

class GFG
{
static int MAX = 26;

// Function to perform the queries
static void performQueries(String str, int q, int []type,
                                     char []ch, int []freq)
{
    int n = str.Length;

    // L[i,j] stores the largest i
    // such that ith character appears
    // exactly jth times in str[0...i]
    int [,]L = new int[MAX, n];

    // F[i,j] stores the smallest i
    // such that ith character appears
    // exactly jth times in str[0...i]
    int [,]F = new int[MAX, n];

    // To store the frequency of each
    // of the character of str
    int []cnt = new int[MAX];
    for (int i = 0; i < n; i++) 
    {

        // Current character of str
        int k = str[i] - 'a';

        // Update its frequency
        cnt[k]++;

        // For every lowercase character
        // of the English alphabet
        for (int j = 0; j < MAX; j++) 
        {

            // If it is equal to the character
            // under consideration then update
            // L[,] and R[,] as it is cnt[j]th
            // occurrence of character k
            if (k == j) 
            {
                L[j, cnt[j]] = i;
                F[j, cnt[j]] = i;
            }

            // Only update L[,] as k has not
            // been occurred so only index
            // has to be incremented
            else
                L[j, cnt[j]] = L[j, cnt[j]] + 1;
        }
    }

    // Perform the queries
    for (int i = 0; i < q; i++) 
    {

        // Type 1 query
        if (type[i] == 1)
        {
            Console.Write(L[ch[i] - 'a', freq[i]]);
        }

        // Type 2 query
        else
        {
            Console.Write(F[ch[i] - 'a', freq[i]]);
        }
        Console.Write("\n");
    }
}

// Driver code
public static void Main(String []args) 
{
    String str = "geeksforgeeks";

    // Queries
    int []type = { 1, 2 };
    char []ch = { 'e', 'k' };
    int []freq = { 2, 2 };
    int q = type.Length;

    // Perform the queries
    performQueries(str, q, type, ch, freq);
}
}

// This code is contributed by 29AjayKumar

JavaScript

<script>

    // JavaScript implementation of the approach
    
    let MAX = 26;
  
    // Function to perform the queries
    function performQueries(str, q, type, ch, freq)
    {
        let n = str.length;

        // L[i][j] stores the largest i
        // such that ith character appears
        // exactly jth times in str[0...i]
        let L = new Array(MAX);

        // F[i][j] stores the smallest i
        // such that ith character appears
        // exactly jth times in str[0...i]
        let F = new Array(MAX);

        // To store the frequency of each
        // of the character of str
        let cnt = new Array(MAX);
        
        for (let i = 0; i < MAX; i++)
        {
            L[i] = new Array(n);
            F[i] = new Array(n);
            cnt[i] = 0;
            for (let j = 0; j < n; j++)
            {
                L[i][j] = 0;
                F[i][j] = 0;
            }
        }
        
        for (let i = 0; i < n; i++) 
        {

            // Current character of str
            let k = str[i].charCodeAt() - 'a'.charCodeAt();

            // Update its frequency
            cnt[k]++;

            // For every lowercase character
            // of the English alphabet
            for (let j = 0; j < MAX; j++) 
            {

                // If it is equal to the character
                // under consideration then update
                // L[][] and R[][] as it is cnt[j]th
                // occurrence of character k
                if (k == j) 
                {
                    L[j][cnt[j]] = i;
                    F[j][cnt[j]] = i;
                }

                // Only update L[][] as k has not
                // been occurred so only index
                // has to be incremented
                else
                    L[j][cnt[j]] = L[j][cnt[j]] + 1;
            }
        }

        // Perform the queries
        for (let i = 0; i < q; i++) 
        {

            // Type 1 query
            if (type[i] == 1)
            {
                document.write(L[ch[i].charCodeAt() - 
                'a'.charCodeAt()][freq[i]]);
            }

            // Type 2 query
            else
            {
                document.write(F[ch[i].charCodeAt() - 
                'a'.charCodeAt()][freq[i]]);
            }
            document.write("</br>");
        }
    }
    
    let str = "geeksforgeeks";
  
    // Queries
    let type = [ 1, 2 ];
    let ch = [ 'e', 'k' ];
    let freq = [ 2, 2 ];
    let q = type.length;
  
    // Perform the queries
    performQueries(str, q, type, ch, freq);

</script>

Output:

8
11

Time Complexity: O(n)
Auxiliary Space: O(26 * n)

Count Occurrences of a Given Character in a String

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Index of character depending on frequency count in string

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