Longest Substring Without Repeating Characters

Topic: 06. Hashing & Strings

Link: LeetCode 3 - Longest Substring

1. Logical Breakdown

• [x] Core Logic: Sliding Window [left, right].
• [x] Constraint: If duplicate found, shrink window from left.

2. Visualization

graph LR Expand["Expand Right"] --> Check["Duplicate in Window?"] Check -- Yes --> Shrink["Remove Left & Increment Left"] Check -- No --> Add["Add Char & Update Max"] Shrink --> Check

3. Complexity

• Time: O(N)
• Space: O(K)

4. Code

package com.dsa.hashing;
import java.util.HashSet;
import java.util.Set;

public class LongestSubstring {
    public int solve(String s) {
        int n = s.length();
        int left = 0, right = 0, maxLen = 0;
        Set<Character> window = new HashSet<>();

        while (right < n) {
            if (!window.contains(s.charAt(right))) {
                window.add(s.charAt(right));
                maxLen = Math.max(maxLen, right - left + 1);
                right++;
            } else {
                window.remove(s.charAt(left));
                left++;
            }
        }
        return maxLen;
    }
}