3. Longest Substring Without Repeating Characters
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.

Intuition

Maintain two pointers left and right. Iterate over array using right pointer. Keep inserting character at right into a map along with index.

If we encounter a duplicate character, move left to current character's index + 1 (or right + 1).

Calculate result as result = Math.max(result, right-left+1); and keep track of it.

Visualisation

Code

class Solution {
    public int lengthOfLongestSubstring(String s) {
        int left=0, result=0;
        Map<Character, Integer> map = new HashMap<>();
        for(int right=0; right<s.length(); right++) {
            char c = s.charAt(right);
            if(map.containsKey(c)) {
                left = Math.max(left, map.get(c) + 1);
            }
            map.put(c, right);
            result = Math.max(result, right-left+1);
        }
        return result;
    }
}

Learning

This problem expects us to know how to:

• Iterate over array and use hash map

• Use two pointers approach

• Calculate substring length using formula (right - left + 1)

• Handle corner case of