Minimum Window Substring in DSA C - Build from Scratch

Minimum Window Substring

📖 Scenario: You are working on a text processing tool that needs to find the smallest part of a text containing all required characters.Imagine you have a long string and a set of characters you want to find inside it. Your task is to find the shortest substring that contains all these characters at least once.

🎯 Goal: Build a program in C that finds the minimum window substring from a given string s that contains all characters of another string t.

📋 What You'll Learn

Create two input strings: s and t with exact values

Create an integer array char_count to count characters needed

Implement the sliding window technique using two pointers start and end

Find and print the minimum window substring that contains all characters of t

If no such window exists, print an empty string

💡 Why This Matters

🌍 Real World

Finding minimum window substrings is useful in text search, DNA sequence analysis, and data filtering where you want the smallest segment containing all required elements.

💼 Career

This problem is common in coding interviews and helps develop skills in string manipulation, hash maps, and two-pointer techniques used in software development.

1

Create input strings

Create two strings called s and t with these exact values: s = "ADOBECODEBANC" and t = "ABC".

DSA C

# Create strings s and t with given values
// Your code here

Hint

Use char s[] = "ADOBECODEBANC"; and char t[] = "ABC"; to create the strings.

2

Initialize character count array

Create an integer array called char_count of size 256 and initialize all elements to 0. Then, fill char_count with the frequency of each character in t using a for loop with variable i.

DSA C

#include 
#include 

char s[] = "ADOBECODEBANC";
char t[] = "ABC";

// Create and initialize char_count array
// Your code here

Hint

Use a loop to count characters in t and store counts in char_count.

3

Implement sliding window to find minimum substring

Implement the sliding window using two integer variables start and end initialized to 0. Use an integer count initialized to the length of t. Use a while loop with condition end < strlen(s). Inside the loop, if char_count[s[end]] > 0, decrement count. Decrement char_count[s[end]] and increment end. When count == 0, move start forward while char_count[s[start]] < 0, incrementing char_count[s[start]] and start. Track the minimum window length and start index.

DSA C

#include 
#include 

char s[] = "ADOBECODEBANC";
char t[] = "ABC";

int char_count[256] = {0};

for (int i = 0; t[i] != '\0'; i++) {
    char_count[(unsigned char)t[i]]++;
}

// Implement sliding window to find minimum substring
// Your code here

Hint

Use two pointers start and end to slide over s. Adjust counts and track minimum window.

4

Print the minimum window substring

Print the minimum window substring using printf. If min_len is greater than the length of s, print an empty string "". Otherwise, print characters from min_start to min_start + min_len - 1.

DSA C

#include 
#include 

char s[] = "ADOBECODEBANC";
char t[] = "ABC";

int char_count[256] = {0};

for (int i = 0; t[i] != '\0'; i++) {
    char_count[(unsigned char)t[i]]++;
}

int start = 0, end = 0, min_start = 0, min_len = 1000000;
int count = strlen(t);

while (end  0) {
        count--;
    }
    char_count[(unsigned char)s[end]]--;
    end++;

    while (count == 0) {
        if (end - start  0) {
            count++;
        }
        start++;
    }
}

// Print the minimum window substring
// Your code here

Hint

Print the substring from min_start to min_start + min_len - 1. If no valid window, print empty quotes.

Practice

1. What is the main goal of the Minimum Window Substring problem?

easy

A. Find the smallest substring in a string that contains all characters of another string

B. Count the total number of substrings in a string

C. Find the longest substring without repeating characters

D. Sort the characters of a string in ascending order

Solution

1. Step 1: Understand the problem statement

   The problem asks to find the smallest part of a string that includes all characters from another string.

2. Step 2: Compare options with the problem goal

   Only Find the smallest substring in a string that contains all characters of another string matches the goal of finding the smallest substring containing all characters of another string.

3. Final Answer:

   Find the smallest substring in a string that contains all characters of another string -> Option A

4. Quick Check:

   Minimum Window Substring = Smallest substring containing all chars [OK]

Hint: Look for smallest substring covering all target chars [OK]

Common Mistakes:

• Confusing with longest substring problems
• Thinking it counts substrings instead of finding minimum
• Mixing sorting with substring search

2. Which of the following C code snippets correctly initializes a frequency array for characters in the Minimum Window Substring problem?

easy

A. int freq[256] = {0};

B. int freq[26]; for(int i=0; i<26; i++) freq[i] = -1;

C. int freq[128]; memset(freq, 1, sizeof(freq));

D. int freq[256]; freq = {0};

Solution

1. Step 1: Check correct array initialization syntax in C

   int freq[256] = {0}; uses correct syntax to initialize all 256 elements to zero using brace initialization.

2. Step 2: Analyze other options for errors

   int freq[26]; for(int i=0; i<26; i++) freq[i] = -1; does not initialize all elements and sets to -1 incorrectly. int freq[128]; memset(freq, 1, sizeof(freq)); uses memset with 1 which sets bytes to 1, not zero. int freq[256]; freq = {0}; tries to assign array which is invalid in C.

3. Final Answer:

   int freq[256] = {0}; -> Option A

4. Quick Check:

   Correct zero initialization = int freq[256] = {0}; [OK]

Hint: Use brace initialization to zero arrays in C [OK]

Common Mistakes:

• Using memset with wrong value
• Trying to assign arrays directly
• Not initializing all elements

3. What will be the output of the following C code snippet for Minimum Window Substring?

char s[] = "ADOBECODEBANC";
char t[] = "ABC";
// Assume function minWindow returns the minimum window substring
char* result = minWindow(s, t);
printf("%s", result);

Assuming minWindow is correctly implemented, what is printed?

medium

A. CODEBA

B. ADOBEC

C. BANC

D. DOBECODEBA

Solution

1. Step 1: Understand the input strings

   String s is "ADOBECODEBANC" and t is "ABC". We want the smallest substring of s containing A, B, and C.

2. Step 2: Identify the minimum window substring

   The substring "BANC" is the smallest substring containing all characters A, B, and C.

3. Final Answer:

   BANC -> Option C

4. Quick Check:

   Minimum window substring for "ABC" in s = "BANC" [OK]

Hint: Remember classic example: "BANC" is minimum window for "ABC" [OK]

Common Mistakes:

• Choosing first substring containing all chars instead of minimum
• Confusing order of characters
• Ignoring repeated characters

4. Consider this buggy C code snippet for Minimum Window Substring:

int left = 0, right = 0;
int count = 0;
while (right < strlen(s)) {
    if (freq[s[right]] > 0) count++;
    freq[s[right]]--;
    right++;
    while (count == t_len) {
        if (right - left < min_len) {
            min_len = right - left;
            start = left;
        }
        freq[s[left]]++;
        if (freq[s[left]] > 0) count--;
        left++;
    }
}
// Returns substring from start with length min_len

What is the main bug in this code?

medium

A. Using strlen(s) inside the loop condition causes repeated length calculation

B. Incorrect update of count when moving left pointer

C. The inner while condition should be count >= t_len

D. freq array is not initialized before use

Solution

1. Step 1: Analyze freq array usage

   The freq array is decremented and incremented but not initialized, so it contains garbage values causing incorrect logic.

2. Step 2: Check other options

   Using strlen(s) inside the loop condition causes repeated length calculation is inefficient but not a bug. Incorrect update of count when moving left pointer logic is correct for count update. The inner while condition should be count >= t_len is incorrect because count == t_len is the correct condition.

3. Final Answer:

   freq array is not initialized before use -> Option D

4. Quick Check:

   freq must be zero-initialized before use [OK]

Hint: Always initialize frequency arrays before use [OK]

Common Mistakes:

• Ignoring initialization of freq array
• Misunderstanding count update logic
• Confusing loop conditions

5. You want to find the minimum window substring of s = "aabbccdd" that contains all characters of t = "abc". Which approach correctly handles repeated characters in s and t to find the minimum window?

hard

A. Count unique characters in t and find first substring in s containing those unique chars

B. Use two frequency arrays: one for t counts and one for current window counts, then expand and contract window accordingly

C. Use a single frequency array for t and move right pointer until all chars found, then stop

D. Sort s and t and find common characters sequentially

Solution

1. Step 1: Understand the need for frequency tracking

   Since t can have repeated characters, we must track how many times each character is needed and how many times it appears in the current window.

2. Step 2: Use two frequency arrays and sliding window

   One array stores counts of characters in t, another tracks counts in the current window. Expand right pointer to include chars, contract left pointer to minimize window while keeping all required chars.

3. Final Answer:

   Use two frequency arrays: one for t counts and one for current window counts, then expand and contract window accordingly -> Option B

4. Quick Check:

   Two freq arrays + sliding window = correct approach [OK]

Hint: Track needed and current counts separately for repeated chars [OK]

Common Mistakes:

• Ignoring repeated characters in t
• Using only unique character counts
• Stopping window expansion too early