LeetCode 3: Longest Substring Without Repeating Characters

Use a sliding window whose left boundary always remains after the previous occurrence of the current character.

import java.util.HashMap;
import java.util.Map;

class Solution {
    public int lengthOfLongestSubstring(String s) {
        Map<Character, Integer> lastIndex = new HashMap<>();
        int left = 0;
        int best = 0;

        for (int right = 0; right < s.length(); right++) {
            char ch = s.charAt(right);
            Integer previous = lastIndex.get(ch);
            if (previous != null && previous >= left) {
                left = previous + 1;
            }
            lastIndex.put(ch, right);
            best = Math.max(best, right - left + 1);
        }

        return best;
    }
}

For abcabcbb, the longest valid window has length three. Updating left with max-equivalent logic prevents the window from moving backward.

Time complexity is O(n) and extra space is O(k), where k is the number of distinct characters tracked. For full Unicode code points rather than UTF-16 code units, iterate with codePoints() or explicit code-point indexes.