Maximum Length Substring With Two Occurrences

Maximum Length Substring With Two Occurrences - Problem

Given a string s, return the maximum length of a substring such that it contains at most two occurrences of each character.

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

Input & Output

Example 1 — Basic Case

$ Input: s = "bcbbbcba"

› Output: 4

💡 Note: The optimal substring is "bcbb" with b appearing 2 times and c appearing 1 time, giving length 4

Example 2 — All Same Character

$ Input: s = "aaaa"

› Output: 2

💡 Note: Since we can have at most 2 occurrences of each character, the longest valid substring is "aa" with length 2

Example 3 — All Different Characters

$ Input: s = "abcde"

› Output: 5

💡 Note: All characters are different, so the entire string is valid with length 5

Constraints

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

Visualization

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Asked in

G Google 15 f Facebook 12 a Amazon 8

The key insight is to use a sliding window approach to maintain a substring where each character appears at most twice. Best approach is sliding window with two pointers. Time: O(n), Space: O(1)

Common Approaches

✓ Brute Force - Check All Substrings

⏱️ Time: O(n³) Space: O(1)

For every starting position, try all possible ending positions. For each substring, count character frequencies to verify no character appears more than twice.

Sliding Window

⏱️ Time: O(n) Space: O(1)

Maintain a sliding window using two pointers. Expand the right pointer to include new characters. When a character count exceeds 2, shrink from the left until the window is valid again.

Brute Force - Check All Substrings — Algorithm Steps

Step 1: Try every possible starting position
Step 2: For each start, try every possible ending position
Step 3: Count character frequencies in each substring
Step 4: Keep track of maximum valid length

Visualization

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Step-by-Step Walkthrough

Generate Substrings

Try all starting and ending positions

Count Characters

For each substring, count character frequencies

Find Maximum

Track the 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++) {
            // Check substring s[i:j+1]
            int charCount[256] = {0}; // ASCII characters
            int valid = 1;
            
            for (int k = i; k <= j; k++) {
                charCount[(unsigned char)s[k]]++;
                if (charCount[(unsigned char)s[k]] > 2) {
                    valid = 0;
                    break;
                }
            }
            
            if (valid) {
                int len = j - i + 1;
                if (len > maxLen) {
                    maxLen = len;
                }
            }
        }
    }
    
    return maxLen;
}

int main() {
    char s[100001];
    fgets(s, sizeof(s), stdin);
    // Remove newline
    s[strcspn(s, "\n")] = 0;
    
    int result = solution(s);
    printf("%d\n", result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n³)

Nested loops to generate O(n²) substrings, O(n) to check each substring

⚠ Quadratic Growth

Space Complexity

O(1)

Only using variables for counting, no extra data structures

✓ Linear Space

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Medium Frequency

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Smart Actions

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Input & Output

Constraints

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Related Problems

Common Approaches

Brute Force - Check All Substrings — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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