3 Longest Substring Without Repeating Characters

To solve a problem of this sort, I must understand the concept of a sliding window . When you have linear data structures to work with like arrays, lists, or strings - you can always maintain a window (fixed or sliding) and move it over the array. Think of it like this - when you are iterating over an array one element at a time, you are (in a way) maintaining a window of n = 1.

Code:

Python

class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:
        left,right,max_len = 0,0,0
        lookup = set()
 
        while right < len(s):
           # check if s[right] is window[left]
           # if yes -> left++
           # if no -> right++ (increase window)
            if s[right] in lookup:
               lookup.remove(s[left])
               left += 1
            else:
                lookup.add(s[right])
                right += 1
            max_len = max(max_len, right-left) 
        return max_len

Big O Analysis

The time complexity here is O(N) as left and right would have to travel to the end of string of length N.

The space complexity is O(N) as well, because the set we are using can increase and decrease with respect to string of length N.

Optimization:

The space can be optimized - use a HashMap or Array to keep running track if we have encountered any alphabets - in our window. This would be constant space O(1) as there are only 26 alphabets = O(26) = O(1)

— A

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