Day 15: Decode String: stack - leetcode - Python3

 Given an encoded string, return its decoded string.

The encoding rule is: k[encoded_string], where the encoded_string inside the square brackets is being repeated exactly k times. Note that k is guaranteed to be a positive integer.

You may assume that the input string is always valid; there are no extra white spaces, square brackets are well-formed, etc. Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, k. For example, there will not be input like 3a or 2[4].

The test cases are generated so that the length of the output will never exceed 105.

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"

Constraints:

• 1 <= s.length <= 30
• s consists of lowercase English letters, digits, and square brackets '[]'.
• s is guaranteed to be a valid input.
• All the integers in s are in the range [1, 300].

SOLUTION:

class Solution:

    def decodeString(self, s: str) -> str:

        stack: List[int] = []

        for i in s:

            if i !=  "]":

                stack.append(i)

            else:

                string: str = ""

                while stack[-1] != "[":

                    string = stack.pop() + string

                stack.pop() #pop "["

               

                k: str = ""

                while stack and stack[-1].isdigit():

                    k = stack.pop() + k

                stack.append(int(k) * string)

        return "".join(stack)

Time Complexity: O(n)

Space Complexity: O(n)

How it works:

1. Initialize an empty stack and iterate over each character i in the input string s.

2. If the current character i is not equal to "]", it means it is a character that needs to be processed. In this case, the character is pushed onto the stack.

3. If the current character i is equal to "]", it indicates that a substring needs to be repeated. The code enters an inner loop.

4. Inside the inner loop, the code pops characters from the stack until it encounters the corresponding opening bracket "[". During this process, each character is concatenated with the string variable in reverse order. This step extracts the substring that needs to be repeated.

5. After the inner loop, the opening bracket "[" is popped from the stack to remove it.

6. Next, another loop is executed while there are characters left in the stack and the top character is a digit. This loop extracts the number of times the substring should be repeated. Each digit character is concatenated with the k variable in reverse order.

7. After the second loop, the code calculates the result of the repetition by multiplying the integer value of k with the string substring.

8. The resulting repeated substring is pushed back onto the stack.

9. Finally, after iterating through all the characters in the input string s, the code joins the elements in the stack to form a single string and returns it as the decoded result.