This problem was recently asked by Microsoft:

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

Here is an example solution in Python language.

class Solution:
  def lengthOfLongestSubstring(self, s):
    # Fill this in.

print Solution().lengthOfLongestSubstring('abrkaabcdefghijjxxx')
# 10

Can you find a solution in linear time?