Longest Substring with At Most Two Distinct Characters

Longest Substring with At Most Two Distinct Characters - Problem

Given a string s, return the length of the longest substring that contains at most two distinct characters.

A substring is a contiguous sequence of characters within a string. For example, in the string "abcdef", the substring "bcd" starts at index 1 and ends at index 3.

Example: For the string "eceba", the longest substring with at most two distinct characters is "ece" which has length 3.

Input & Output

Example 1 — Basic Case

$ Input: s = "eceba"

› Output: 3

💡 Note: The longest substring with at most 2 distinct characters is "ece" with length 3

Example 2 — All Same Characters

$ Input: s = "aaaa"

› Output: 4

💡 Note: The entire string has only 1 distinct character, so the answer is 4

Example 3 — Two Characters Mixed

$ Input: s = "abacbc"

› Output: 4

💡 Note: The longest substring is "abac" or "acbc", both with 2 distinct characters and length 4

Constraints

1 ≤ s.length ≤ 10⁵
s consists of English letters

Visualization

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Asked in

G Google 45 f Facebook 38 a Amazon 32 M Microsoft 28

The key insight is to use a sliding window with hash map to efficiently track character frequencies. The optimal sliding window approach maintains at most 2 distinct characters by expanding the right pointer and contracting the left pointer when needed. Time: O(n), Space: O(1)

Common Approaches

✓ Brute Force

⏱️ Time: O(n³) Space: O(1)

Generate every possible substring and check if it contains at most 2 distinct characters. Keep track of the maximum length found.

Sliding Window with Hash Map

⏱️ Time: O(n) Space: O(1)

Maintain a sliding window using two pointers (left and right) and a hash map to count character frequencies. Expand the right pointer to include new characters, and contract the left pointer when we have more than 2 distinct characters.

Brute Force — Algorithm Steps

For each starting position
For each ending position
Count distinct characters in substring
Update maximum if valid

Visualization

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Step-by-Step Walkthrough

Generate Substrings

Create all possible substrings using nested loops

Count Characters

For each substring, count distinct characters

Track Maximum

Keep track of longest valid substring

Code -

solution.c — C

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

int solution(char* s) {
    int n = strlen(s);
    int maxLen = 0;
    
    for (int i = 0; i < n; i++) {
        for (int j = i; j < n; j++) {
            // Count distinct characters in substring s[i:j+1]
            int charCount[128] = {0}; // ASCII characters
            int distinctCount = 0;
            
            for (int k = i; k <= j; k++) {
                if (charCount[s[k]] == 0) {
                    distinctCount++;
                }
                charCount[s[k]]++;
            }
            
            // If at most 2 distinct characters, update maxLen
            if (distinctCount <= 2) {
                int currentLen = j - i + 1;
                if (currentLen > maxLen) {
                    maxLen = currentLen;
                }
            }
        }
    }
    
    return maxLen;
}

int main() {
    char s[10001];
    fgets(s, sizeof(s), stdin);
    s[strcspn(s, "\n")] = 0; // Remove newline
    printf("%d\n", solution(s));
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n³)

Nested loops generate O(n²) substrings, each requiring O(n) to count distinct characters

✓ Linear Growth

Space Complexity

O(1)

Only using a few variables for counting, no extra data structures

✓ Linear Space

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Input & Output

Constraints

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Related Problems

Common Approaches

Brute Force — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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