Python: Count number of substrings from a given string of lowercase alphabets with exactly k distinct characters

Last update on June 12 2025 12:42:56 (UTC/GMT +8 hours)

Substrings with k distinct characters.

Write a Python program to count the number of substrings from a given string of lowercase alphabets with exactly k distinct (given) characters.

Sample Solution:

Python Code:

# Function to count substrings with k distinct chars
def count_k_dist(str1, k):

  # Get string length
  str_len = len(str1)
  
  result = 0

  # Character counter array
  ctr = [0] * 27

  for i in range(0, str_len):

    dist_ctr = 0
    ctr = [0] * 27

    for j in range(i, str_len):
      
      # Increment distinct char count
      if(ctr[ord(str1[j]) - 97] == 0):
        dist_ctr += 1

      # Increment char counter
      ctr[ord(str1[j]) - 97] += 1

      # Increment result if exactly k distinct
      if(dist_ctr == k):
        result += 1

      # Break loop if > k distinct  
      if(dist_ctr > k):
        break

  return result

# Get input  
str1 = input("Input a string (lowercase alphabets):")
k = int(input("Input k: "))

# Print result
print("Number of substrings with exactly", k, "distinct characters : ", end = "")
print(count_k_dist(str1, k)) 

Sample Output:

Input a string (lowercase alphabets): wolf
Input k:  4
Number of substrings with exactly 4 distinct characters : 1

Flowchart:

For more Practice: Solve these Related Problems:

• Write a Python program to count the number of substrings in a string that contain exactly k distinct characters using a sliding window approach.
• Write a Python program to use nested loops and a set to generate and count substrings with exactly k distinct letters.
• Write a Python program to implement a function that returns the count of substrings with k unique characters with optimized time complexity.
• Write a Python program to use dictionary frequency counting to track distinct characters and count valid substrings of a given length.

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