Longest Substring Without Repeating Characters (LeetCode)

Problem

Given a string s, find the length of the longest 

substring without repeating characters.

Example 1:

Input: s = “abcabcbb”

Output: 3

Explanation: The answer is “abc”, with the length of 3.

Example 2:

Input: s = “bbbbb”

Output: 1

Explanation: The answer is “b”, with the length of 1.

Example 3:

Input: s = “pwwkew”

Output: 3

Explanation: The answer is “wke”, with the length of 3. Notice that the answer must be a substring, “pwke” is a subsequence and not a substring.

Constraints:

• 0 <= s.length <= 5 * 104
• s consists of English letters, digits, symbols and spaces.

Solution

public int LengthOfLongestSubstring(string s) {
        var longest = 0;
	    var start = 0; // searching for sequence from
	   
        for (var i = 0; i < s.Length; i++) {
            // Find the index of current i character from start till next i-start characters.
            // e.g. defabcab
            // In above example 'a' is repeated in first unique sequence (defabc) at index 6. 
            // So we know for sure that next non repeating sequence could start from 4th index.
            // What we do is, first we find the occurance of current ith character in from start till i
            // If we find don't find one then we check if sequence is longer. If yes, then we update the longest
            // and if find the ith character to be repeated, 
            // then we start from next character of the repeating character in the current unique sequence
			var indexOfCurrentChar = s.IndexOf(s[i], start, i - start); 
            if (indexOfCurrentChar == -1) {
                if (longest < i - start + 1) {
                    longest = i - start + 1;
                }
            } else {
                // next seq start would be from the next character which is repeated
				start = indexOfCurrentChar + 1;
            }
        }
        return longest;
    }