Leetcode – Find Lucky Integer in an Array Solution

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The problem “Find Lucky Integer in an Array” is one of those challenges that might appear simple at first glance but can reveal deeper insights into the Python language, algorithms, and data structures if examined carefully. In this extensive article, we’ll delve deep into this problem, analyzing multiple ways to solve it and discussing their advantages and disadvantages.

Table of Contents

  1. Understanding the Problem Statement
  2. Breaking Down the Problem
  3. Brute-Force Approach
  4. Using a Dictionary for Frequency Counting
  5. Using Python Collections: Counter
  6. Time and Space Complexity Analysis
  7. Testing and Edge Cases
  8. Alternative Approaches
  9. Conclusion

1. Understanding the Problem Statement

The problem asks you to find the “lucky integer” in a given array. An integer is considered “lucky” if the frequency of that integer in the array is the same as the integer value itself. You have to return the largest lucky integer, or -1 if there’s no such integer.

Constraints and Assumptions

  • The given array contains n integers, where 1 <= n <= 500.
  • Each integer in the array will be in the range of 1 <= arr[i] <= 500.

2. Breaking Down the Problem

At the core, this problem is about counting the frequency of each integer in the array and then finding the largest one that satisfies the “lucky” condition.

3. Brute-Force Approach

The most naive way to solve this problem is to loop through each unique integer in the array, count its occurrences, and check whether it is a “lucky” integer.

Python Code Implementation:

def findLucky(arr):
    max_lucky = -1
    for num in set(arr):
        if arr.count(num) == num:
            max_lucky = max(max_lucky, num)
    return max_lucky

4. Using a Dictionary for Frequency Counting

A better approach would be to use a dictionary to store the frequency of each number, thereby reducing the number of times you have to loop through the array.

Python Code Implementation:

def findLucky(arr):
    freq_dict = {}
    for num in arr:
        if num in freq_dict:
            freq_dict[num] += 1
        else:
            freq_dict[num] = 1
    
    max_lucky = -1
    for num, freq in freq_dict.items():
        if num == freq:
            max_lucky = max(max_lucky, num)
    
    return max_lucky

5. Using Python Collections: Counter

Python’s standard library offers a Counter class in the collections module, which can simplify the frequency counting part of the code.

Python Code Implementation:

from collections import Counter

def findLucky(arr):
    freq_counter = Counter(arr)
    max_lucky = -1
    for num, freq in freq_counter.items():
        if num == freq:
            max_lucky = max(max_lucky, num)
    
    return max_lucky

6. Time and Space Complexity Analysis

  • Brute-Force Approach: Time Complexity is O(n^2) and Space Complexity is O(n).
  • Using a Dictionary for Frequency Counting: Time Complexity is O(n) and Space Complexity is O(n).
  • Using Python Collections: Counter: Time Complexity is O(n) and Space Complexity is O(n).

7. Testing and Edge Cases

Be sure to test your solution against various cases:

  • Arrays with no lucky integers.
  • Arrays with multiple lucky integers.
  • Arrays with duplicate lucky integers.
  • Arrays with a single element.

8. Alternative Approaches

One could also sort the array and count frequencies, but given that Python’s Counter is so efficient, this is usually not necessary.

9. Conclusion

The “Find Lucky Integer in an Array” problem serves as a useful exercise for practicing frequency counting and the use of dictionaries in Python. Even though the problem might seem simple at first glance, the different approaches reveal a lot about Python’s capabilities and the importance of knowing when to use what data structure for optimal performance.

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