Python: Find smallest window that contains all characters of a given string

Last update on June 12 2025 12:42:55 (UTC/GMT +8 hours)

Smallest window with all characters.

Write a Python program to find the smallest window that contains all characters in a given string.

Visual Presentation:

Sample Solution:

Python Code:

from collections import defaultdict   

def find_sub_string(str): 
    str_len = len(str) 
      
    # Count all distinct characters. 
    dist_count_char = len(set([x for x in str])) 
  
    ctr, start_pos, start_pos_index, min_len = 0, 0, -1, 9999999999
    curr_count = defaultdict(lambda: 0) 
    for i in range(str_len): 
        curr_count[str[i]] += 1
 
        if curr_count[str[i]] == 1: 
            ctr += 1
  
        if ctr == dist_count_char: 
            while curr_count[str[start_pos]] > 1: 
                if curr_count[str[start_pos]] > 1: 
                    curr_count[str[start_pos]] -= 1
                start_pos += 1
  
            len_window = i - start_pos + 1
            if min_len > len_window: 
                min_len = len_window 
                start_pos_index = start_pos 
    return str[start_pos_index: start_pos_index + min_len] 
      
str1 = "asdaewsqgtwwsa"
print("Original Strings:\n",str1)
print("\nSmallest window that contains all characters of the said string:")
print(find_sub_string(str1))  

Sample Output:

Original Strings:
 asdaewsqgtwwsa

Smallest window that contains all characters of the said string:
daewsqgt

Flowchart:

For more Practice: Solve these Related Problems:

• Write a Python program to compute the smallest window in a string that contains all unique characters from the string.
• Write a Python program to implement a sliding window algorithm to find the minimal substring that contains all characters of the original string.
• Write a Python program to use collections.Counter and two pointers to determine the smallest window containing every character.
• Write a Python program to build a function that returns the smallest substring with all unique letters of the input string.

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