Introduction

In this blog post, we will explore how to solve the LeetCode problem "1946" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like Chatmagic, we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

Problem Statement

You are given a string num, which represents a large integer. You are also given a 0-indexed integer array change of length 10 that maps each digit 0-9 to another digit. More formally, digit d maps to digit change[d]. You may choose to mutate a single substring of num. To mutate a substring, replace each digit num[i] with the digit it maps to in change (i.e. replace num[i] with change[num[i]]). Return a string representing the largest possible integer after mutating (or choosing not to) a single substring of num. A substring is a contiguous sequence of characters within the string. Example 1: Input: num = "132", change = [9,8,5,0,3,6,4,2,6,8] Output: "832" Explanation: Replace the substring "1": - 1 maps to change[1] = 8. Thus, "132" becomes "832". "832" is the largest number that can be created, so return it. Example 2: Input: num = "021", change = [9,4,3,5,7,2,1,9,0,6] Output: "934" Explanation: Replace the substring "021": - 0 maps to change[0] = 9. - 2 maps to change[2] = 3. - 1 maps to change[1] = 4. Thus, "021" becomes "934". "934" is the largest number that can be created, so return it. Example 3: Input: num = "5", change = [1,4,7,5,3,2,5,6,9,4] Output: "5" Explanation: "5" is already the largest number that can be created, so return it. Constraints: 1 <= num.length <= 105 num consists of only digits 0-9. change.length == 10 0 <= change[d] <= 9

Explanation

Here's the breakdown of the solution:

• Identify Mutation Range: Iterate through the string num. The goal is to find the first position where a digit can be improved by applying the change mapping. Once found, continue mutating as long as the mutated digit is greater than or equal to the original digit.
• Apply Mutation: Mutate the identified substring using the change mapping.

• Handle No Mutation: If no digit can be improved, return the original num.

• Runtime Complexity: O(n), where n is the length of the input string num.

• Storage Complexity: O(n) due to the string operations (creating new strings, which could, in the worst case, require additional storage proportional to the size of the original string).

Code

    def maximum_mutated_string(num: str, change: list[int]) -> str:
    """
    Finds the largest possible integer after mutating a single substring of num.

    Args:
        num: A string representing a large integer.
        change: A 0-indexed integer array of length 10 that maps each digit 0-9 to another digit.

    Returns:
        A string representing the largest possible integer after mutating (or choosing not to) a single substring of num.
    """

    n = len(num)
    start = -1
    end = -1

    for i in range(n):
        digit = int(num[i])
        if change[digit] > digit:
            start = i
            break

    if start == -1:
        return num

    end = start
    for i in range(start, n):
        digit = int(num[i])
        if change[digit] >= digit:
            end = i
        else:
            break

    mutated_num = ""
    for i in range(n):
        if start <= i <= end:
            digit = int(num[i])
            mutated_num += str(change[digit])
        else:
            mutated_num += num[i]

    return mutated_num