Decode String Leetcode Problem 394 [Python Solution]

In this blog post, we will explore the Decode String Leetcode problem, falling under the category of Stack.

The goal of this problem is to take an encoded string as input and return its decoded string according to certain rules.

The encoding rule is as follows:

An integer k, representing how many times the encoded string inside the square brackets is being repeated.
The encoded string inside the square brackets, which needs to be repeated exactly k times.

Here are a few examples to illustrate the problem:

Example 1:

Input: s = “3[a]2[bc]”
Output: “aaabcbc”

Example 2:

Input: s = “3[a2[c]]”
Output: “accaccacc”

Example 3:

Input: s = “2[abc]3[cd]ef”
Output: “abcabccdcdcdef”

The problem comes with some constraints:

The input string is guaranteed to be valid, with well-formed square brackets.
There are no extra white spaces in the input string.
Digits in the input string represent the number of times to repeat the characters in square brackets.
The length of the output string will never exceed 105.

Now that we have a clear understanding of the problem, let’s explore the solution in detail.

Understanding the Constraints

Before diving into the solution, let’s understand the constraints provided with the problem.

The input string can have a maximum length of 30 characters, which means we don’t have to worry about very long input strings.
The characters in the input string are limited to lowercase English letters, digits, and square brackets ‘[]’.
The integers in the input string fall within the range [1, 300].

This implies that the repetition factor k will always be a positive integer between 1 and 300.

These constraints give us an idea of the input size and the range of values we need to consider in our solution.

Decode String LeetCode Problem Solution

To solve this problem efficiently, we will use a stack data structure.

We’ll iterate through the characters of the input string one by one, maintaining a stack to keep track of our progress.

Let’s break down the problem step by step:

1. Bruteforce Approach

The brute force approach to solving this problem would be to simulate the decoding process as described in the problem statement.

We can iterate through the input string character by character, maintaining a stack to keep track of the current state.

def decodeString(s: str) -> str:
    stack = []

    for char in s:
        if char is not "]":
            stack.append(char)
        else:
            sub_str = ""
            while stack[-1] is not "[":
                sub_str = stack.pop() + sub_str
            stack.pop()

            multiplier = ""
            while stack and stack[-1].isdigit():
                multiplier = stack.pop() + multiplier

            stack.append(int(multiplier) * sub_str)

    return "".join(stack)

In this brute force approach, we use two nested loops to extract the encoded string inside the square brackets and the multiplier (k) value.

This solution works but is not the most efficient way to solve the problem, especially for large input strings.

2. Efficient Approach

A more efficient approach to solving the problem is to use a single stack to maintain the intermediate results.

We’ll iterate through the input string, and when we encounter a closing bracket, we’ll pop the stack to extract the substring and the multiplier.

Then, we’ll push the evaluated substring back onto the stack.

This approach eliminates the need for nested loops and is more efficient.

def decodeString(s: str) -> str:
    stack = []

    for char in s:
        if char != "]":
            stack.append(char)
        else:
            sub_str = ""
            while stack[-1] != "[":
                sub_str = stack.pop() + sub_str
            stack.pop()

            multiplier = ""
            while stack and stack[-1].isdigit():
                multiplier = stack.pop() + multiplier

            stack.append(int(multiplier) * sub_str)

    return "".join(stack)

This efficient approach reduces the time complexity and is a better solution for the problem.

It eliminates the need for nested loops, making it faster and more straightforward to implement.

Python Code Solution

Here’s the Python code solution for the Decode String problem using the efficient approach:

View on GitHub ➹

def decodeString(s: str) -&gt; str:
    stack = []

    for char in s:
        if char != "]":
            stack.append(char)
        else:
            sub_str = ""
            while stack[-1] != "[":
                sub_str = stack.pop() + sub_str
            stack.pop()

            multiplier = ""
            while stack and stack[-1].isdigit():
                multiplier = stack.pop() + multiplier

            stack.append(int(multiplier) * sub_str)

    return "".join(stack)

This code efficiently decodes the input string following the rules and constraints of the problem.

Time and Space Complexity

Now, let’s analyze the time and space complexity of our efficient solution.

Time Complexity: The time complexity of this solution is O(n), where n is the length of the input string.

We iterate through the input string once, and for each character, we perform constant time operations.

Space Complexity: The space complexity is also O(n) in the worst case.

We use a stack to store characters, and in the worst case, the stack may contain all characters of the input string.

Reasoning Behind Our Approach

The reasoning behind our approach is to efficiently process the input string by using a single stack.

We take advantage of the stack’s last-in, first-out (LIFO) property to handle the nested structure of the encoded string.

When we encounter a closing bracket, we pop the stack to extract the substring and the multiplier, and then we push the evaluated substring back onto the stack.

This approach simplifies the decoding process and ensures that we get the correct output string while adhering to the problem’s rules and constraints.

By using this approach, we eliminate the need for nested loops and make the code more efficient and straightforward.

Conclusion

In this blog post, we explored the Decode String problem on LeetCode.

We discussed the problem statement, examples, constraints, and an efficient Python solution.

Our efficient solution uses a single stack to process the input string, following the encoding rules and returning the decoded string.

To summarize, this problem tests your understanding of stack data structures and your ability to efficiently decode encoded strings.

It’s a valuable exercise in algorithmic thinking and problem-solving.

If you found this blog post helpful, please consider sharing it with others who might benefit from it.

If you have any questions or suggestions, feel free to leave a comment and engage with the community.

We encourage you to explore more coding challenges and keep honing your problem-solving skills.

For more information and to practice the problem, you can visit the Decode String problem on LeetCode.

Happy coding!