3. Longest Substring Without Repeating Characters

Given a string, find the length of the longest substring without repeating characters.

Example 1:

Input: "abcabcbb"
Output: 3 
Explanation: The answer is "abc", with the length of 3. 

Example 2:

Input: "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

Example 3:

Input: "pwwkew"
Output: 3
Explanation: The answer is "wke", with the length of 3. 
             Note that the answer must be a substring, "pwke" is a subsequence and not a substring.

Idea

1. Creat a dictionary window = {char : pos} to record the position of the occurrance of char.
2. For character c in s:
   • If c not in window: window[c] = i
   • Else:

     1. Update all characters from left to window[c] to -1

     2. Update ans

     3. Make left = window[c] + 1

     4. window[c] = i