Idea

• Use hashmap to keep track the occurrence of each character in the desired string
• Use sliding window to find the the minimum window sub-string
  • Keep expanding window when substring isn’t found
  • Keep shrinking window to find minimal when the desired string is found in the substring window
• RMB ❗❗❗
  • HashMap returns Integer object, so we need to use equals() to compare using values instead of object address

Space & time analysis

• Space O(1), because there are only 26 characters
• Time O(n), in worst case, we need to process each character twice (add to window & remove from window)

Code

• 1st attempt (java)
• 2nd attempt (java) ❤️
• 3rd attempt (java) ❤️❤️ (cleaner, but less space efficient than 2nd attempt)