Longest substring without repeating characters
==============================================

input "abcabcbb"
  right=0 'a'  left=0  window="a"  best=1
  right=1 'b'  left=0  window="ab"  best=2
  right=2 'c'  left=0  window="abc"  best=3
  right=3 'a'  left=1  window="bca"  best=3  seen at 0 inside the window, left 0 -> 1
  right=4 'b'  left=2  window="cab"  best=3  seen at 1 inside the window, left 1 -> 2
  right=5 'c'  left=3  window="abc"  best=3  seen at 2 inside the window, left 2 -> 3
  right=6 'b'  left=5  window="cb"  best=3  seen at 4 inside the window, left 3 -> 5
  right=7 'b'  left=7  window="b"  best=3  seen at 6 inside the window, left 5 -> 7
  answer = 3

input "abba"
  right=0 'a'  left=0  window="a"  best=1
  right=1 'b'  left=0  window="ab"  best=2
  right=2 'b'  left=2  window="b"  best=2  seen at 1 inside the window, left 0 -> 2
  right=3 'a'  left=2  window="ba"  best=2  seen at 0 but that is left of the window, left stays 2
  answer = 2