Longest Substring Without Repeating Characters

LongestSubstringWithoutRepeatingCharacters.java

/**
 * Given a string, find the length of the longest substring without repeating characters.
 * For example, the longest substring without repeating letters for "abcabcbb" is "abc", which the length is 3. For "bbbbb" the longest substring is "b", with the length of 1.
 */

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        int n = s.length();
        int i = 0, j = 0;
        int maxLen = 0;
        boolean[] exists = new boolean[256];
        while(j < n){
            if(exists[s.charAt(j)]){
                // met the repeat character, update the maxlen;
                maxLen = Math.max(maxLen, j - i);
                // update pointer i
                while(s.charAt(i) != s.charAt(j)){
                    exists[s.charAt(i)] = false;
                    i++;
                }
                i++;
                j++;
            } else {
                exists[s.charAt(j)] = true;
                j++;
            }
        }
        maxLen = Math.max(maxLen, n - i);
        return maxLen; 
    }
    
    public int lengthOfLongestSubstring(String s) {
        int i = 0, j = 0, max = 0;
        Set<Character> set = new HashSet<>();
    
        while (j < s.length()) {
            if (!set.contains(s.charAt(j))) {
                set.add(s.charAt(j++));
                max = Math.max(max, set.size());
            } else {
                set.remove(s.charAt(i++));
            }
        }
    
        return max;
    }
    
    public int lengthOfLongestSubstring(String s) {
        boolean[] exist = new boolean[256];
        int i = 0, maxLen = 0;
        for(int j = 0; j < s.length(); j++) {
            while(exist[s.charAt(j)]) {
                exist[s.charAt(i)] = false;
                i++;
            }
            exist[s.charAt(j)] = true;
            maxLen = Math.max(maxLen, j - i + 1);
        }
        return maxLen;
    }
}

how does the second solution is work, wouldn't it get stuck in a forever while loop?

elegent solution!