How to Get Longest Substring Without Repeating Characters in Python

1. Longest Substring Without Repeating Characters in Python
2. Use a While Loop to Get the Longest Substring in Python
3. Use a For Loop and an If-Else Statement to Get the Longest Substring in Python

We will introduce how to make substrings in Python and how we can make a substring without repeating characters with examples.

Longest Substring Without Repeating Characters in Python

In this tutorial, we will learn something unique that can boost your knowledge and make your loops even more perfect. We will learn how to make substrings from a string without repeating characters.

The substring means that if we have a string and break it into multiple small parts, they will be called substrings of that specific string. The longest substring method is very helpful if we want to get the longest substring that does not repeat the same alphabets.

In this method, we will use a loop that goes through the complete string, checks each element one by one, and gives the non-repeated substring of the original string.

In Python programming, there are many methods of getting the longest substring. These methods are very simple and helpful; we will discuss them in detail.

Use a While Loop to Get the Longest Substring in Python

We will create a class GetLongestSubstring that will take object as a parameter in this method. We will define a function called Length that will take two parameters inside this class.

We will go through the while loop until we have found the longest substring from the given string.

# python
class GetLongestSubstring(object):
    def Length(self, x):
        a = 0
        b = 0
        c = {}
        sol = 0
        while b < len(x):
            if x[b] not in c or a > c[x[b]]:
                sol = max(sol, (b - a + 1))
                c[x[b]] = b
            else:
                a = c[x[b]] + 1
                sol = max(sol, (b - a + 1))
                b -= 1
            b += 1
        return sol


obj = GetLongestSubstring()
print(obj.Length("ABCDEFGABCGHIJKLMNO"))

Output:

As you can see from the above example, the longest substring possible has a length of 12, the same as the ABCGHIJKLMNO substring from the original string.

Use a For Loop and an If-Else Statement to Get the Longest Substring in Python

We will try to achieve the same scenario using the for loop. First, we will define the function GetLongestSubstring as shown below.

This function will convert the original string into multiple substrings and check if the substring is unique or not. As shown below, this function will return the longest substring with its length.

# python
def GetLongestSubstring(x):
    if len(set(x)) == len(x):
        return len(x)
    Substring = ""
    StrLen = 1
    for a in x:
        if a not in Substring:
            Substring = Substring + a
            StrLen = max(StrLen, len(Substring))
        else:
            Substring = Substring.split(a)[1] + a
    print(Substring)
    return StrLen


print(GetLongestSubstring("ABCDEFGABCGHIJKLMNO"))

Output:

As you can see from the above solution, we can easily get the longest substring from a string using the for loop with the if-else statement.