3. Longest Substring Without Repeating Characters

class Solution:
    def lengthOfLongestSubstring(self, s):
          dict = {}
          ans = 0
          start = 0
          for i, c in enumerate(s):
              if c in dict:
                  start = max(start, dict[c]+1)
              dict[c] = i
              ans = max(ans, i-start+1)
          return ans