Count substrings that starts with character X and ends with character Y

Given a string str of n lowercase characters, the task is to count the number of substrings of str starting with character X and ending with character Y.

Examples:

Input: str = "abbcaceghcak"
        x = 'a', y = 'c'
Output: 5
abbc, abbcac, ac, abbcaceghc, aceghc

Input: str = "geeksforgeeks"
        x = 'g', y = 'e'
Output: 6

Approach:



  • Initialize two counters i.e. tot_count to count the total number of substrings and count_x to count the number of strings that start with X.
  • Start traversing the string.
  • If the current character is X then increment the count of count_x.
  • If the current character is Y, it means a string ends at Y so increment the count of tot_count i.e.
    tot_count = tot_count + count_x

    It means that if there exists a Y then it will make a substring with all the X occurs before Y in the string. So, add the count of X to the total count.

  • Return total count.

Below is the implementation of above approach:

C++

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// C++ implementation to count substrings
// starting with character X and ending
// with character Y
#include <bits/stdc++.h>
using namespace std;
  
// function to count substrings starting with
// character X and ending with character Y
int countSubstr(string str, int n,
                char x, char y)
{
    // to store total count of
    // required substrings
    int tot_count = 0;
  
    // to store count of character 'x'
    // up to the point the string 'str'
    // has been traversed so far
    int count_x = 0;
  
    // traverse 'str' form left to right
    for (int i = 0; i < n; i++) {
  
        // if true, increment 'count_x'
        if (str[i] == x)
            count_x++;
  
        // if true accumulate 'count_x'
        // to 'tot_count'
        if (str[i] == y)
            tot_count += count_x;
    }
  
    // required count
    return tot_count;
}
  
// Driver code
int main()
{
    string str = "abbcaceghcak";
    int n = str.size();
    char x = 'a', y = 'c';
  
    cout << "Count = "
         << countSubstr(str, n, x, y);
  
    return 0;
}

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Java

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// Java implementation to count 
// substrings starting with 
// character X and ending
// with character Y
import java.util.*;
import java.lang.*;
import java.io.*;
  
class GFG
{
// function to count substrings 
// starting with character X and
// ending with character Y
static int countSubstr(String str, int n,
                       char x, char y)
{
    // to store total count of
    // required substrings
    int tot_count = 0;
  
    // to store count of character 
    // 'x' up to the point the 
    // string 'str' has been
    // traversed so far
    int count_x = 0;
  
    // traverse 'str' form
    // left to right
    for (int i = 0; i < n; i++)
    {
  
        // if true, increment 'count_x'
        if (str.charAt(i) == x)
            count_x++;
  
        // if true accumulate 'count_x'
        // to 'tot_count'
        if (str.charAt(i) == y)
            tot_count += count_x;
    }
  
    // required count
    return tot_count;
}
  
// Driver code
public static void main(String args[])
{
    String str = "abbcaceghcak";
    int n = str.length();
    char x = 'a', y = 'c';
  
    System.out.print ("Count = " +
       countSubstr(str, n, x, y));
}
}
  
// This code is contributed
// by Subhadeep

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Python3

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# Python 3 implementation to count substrings 
# starting with character X and ending 
# with character Y 
  
# function to count substrings starting with 
# character X and ending with character Y 
def countSubstr(str, n, x, y):
  
    # to store total count of 
    # required substrings 
    tot_count = 0
  
    # to store count of character 'x' 
    # up to the point the string 'str' 
    # has been traversed so far 
    count_x = 0
  
    # traverse 'str' form left to right
    for i in range(n):
  
        # if true, increment 'count_x' 
        if str[i] == x:
            count_x += 1
  
        # if true accumulate 'count_x' 
        # to 'tot_count' 
        if str[i] == y:
            tot_count += count_x
      
    # required count
    return tot_count
  
# Driver Code
str = 'abbcaceghcak'
n = len(str)
x, y = 'a', 'c'
print('Count =', countSubstr(str, n, x, y))
  
# This code is contributed SamyuktaSHegde

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C#

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// C# implementation to count substrings 
// starting with character X and ending
// with character Y
using System;
  
class GFG
{
// function to count substrings starting 
// with character X and ending with character Y
static int countSubstr(string str, int n,
                       char x, char y)
{
    // to store total count of
    // required substrings
    int tot_count = 0;
  
    // to store count of character 'x' up 
    // to the point the string 'str' has 
    // been traversed so far
    int count_x = 0;
  
    // traverse 'str' form left to right
    for (int i = 0; i < n; i++)
    {
  
        // if true, increment 'count_x'
        if (str[i] == x)
            count_x++;
  
        // if true accumulate 'count_x'
        // to 'tot_count'
        if (str[i] == y)
            tot_count += count_x;
    }
  
    // required count
    return tot_count;
}
  
// Driver code
public static void Main()
{
    string str = "abbcaceghcak";
    int n = str.Length;
    char x = 'a', y = 'c';
  
    Console.Write("Count = " +
    countSubstr(str, n, x, y));
}
}
  
// This code is contributed
// by Akanksha Rai

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PHP

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<?php
// PHP implementation to count substrings
// starting with character X and ending
// with character Y
  
// function to count substrings starting with
// character X and ending with character Y
function countSubstr($str, $n, $x, $y)
{
    // to store total count of
    // required substrings
    $tot_count = 0;
  
    // to store count of character 'x'
    // up to the point the string 'str'
    // has been traversed so far
    $count_x = 0;
  
    // traverse 'str' form left to right
    for ($i = 0; $i < $n; $i++)
    {
  
        // if true, increment 'count_x'
        if ($str[$i] == $x)
            $count_x++;
  
        // if true accumulate 'count_x'
        // to 'tot_count'
        if ($str[$i] == $y)
            $tot_count += $count_x;
    }
  
    // required count
    return $tot_count;
}
  
// Driver code
$str = "abbcaceghcak";
$n = strlen($str);
$x = 'a'; $y = 'c';
  
echo "Count = ". countSubstr($str, $n, $x, $y);
  
// This code is contributed 
// by Akanksha Rai

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Output:

Count = 5


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