Substrings of Size Three with Distinct Characters

Substrings of Size Three with Distinct Characters - Problem

A string is good if there are no repeated characters.

Given a string s, return the number of good substrings of length three in s.

Note that if there are multiple occurrences of the same substring, every occurrence should be counted.

A substring is a contiguous sequence of characters in a string.

Input & Output

Example 1 — Basic Case

$ Input: s = "xyzzaz"

› Output: 1

💡 Note: Only "xyz" at the beginning has all distinct characters. "yzz", "zza", and "zaz" all have repeated characters.

Example 2 — Multiple Good Substrings

$ Input: s = "aababcabc"

› Output: 4

💡 Note: The good substrings are: "bab" (index 2), "abc" (index 3), "bca" (index 4), "cab" (index 5), "abc" (index 6). Total = 4 distinct character substrings.

Example 3 — No Good Substrings

$ Input: s = "aaa"

› Output: 0

💡 Note: Only one substring "aaa" exists, but it has repeated characters, so no good substrings.

Constraints

1 ≤ s.length ≤ 100
s consists of lowercase English letters.

Visualization

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Asked in

a Amazon 25 f Facebook 18 G Google 15 M Microsoft 12

The key insight is that we only need to check if all 3 characters in each substring are distinct by comparing pairs. Best approach is sliding window for clean implementation. Time: O(n), Space: O(1).

Common Approaches

✓ Brute Force - Check Every Substring

⏱️ Time: O(n) Space: O(1)

For each possible starting position, extract a 3-character substring and check if all characters are unique by comparing each character with every other character in the substring.

Sliding Window - Optimized Traversal

⏱️ Time: O(n) Space: O(1)

Slide a window of size 3 through the string, checking if all characters in the current window are distinct. This avoids recreating substrings and focuses on the core logic.

Brute Force - Check Every Substring — Algorithm Steps

Iterate through each possible starting position (0 to n-3)
Extract substring of length 3
Check if all 3 characters are distinct
Count valid substrings

Visualization

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Step-by-Step Walkthrough

Slide Window

Move through each position checking 3 characters

Check Distinct

Compare all pairs to ensure no duplicates

Count Valid

Increment counter for good substrings

Code -

solution.c — C

#include <stdio.h>
#include <string.h>

int solution(char* s) {
    int count = 0;
    int n = strlen(s);
    
    // Check each possible substring of length 3
    for (int i = 0; i <= n - 3; i++) {
        // Check if all characters are distinct
        if (s[i] != s[i+1] && s[i+1] != s[i+2] && s[i] != s[i+2]) {
            count++;
        }
    }
    
    return count;
}

int main() {
    char s[1001];
    fgets(s, sizeof(s), stdin);
    s[strcspn(s, "\n")] = 0;
    int result = solution(s);
    printf("%d\n", result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Single pass through string with constant work per position (checking 3 characters)

✓ Linear Growth

Space Complexity

O(1)

Only using variables for counting, no extra data structures

✓ Linear Space

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Input & Output

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Related Problems

Common Approaches

Brute Force - Check Every Substring — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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