🏯Beginner-Friendly Guide "Find Lucky Integer in an Array" – LeetCode 1394 (C++ | Python | JavaScript)

#programming #cpp #javascript #python

Hey array detectives! 🔍

Today we’re tackling a charming problem that blends frequency counting and a bit of luck. The idea? Identify integers in an array whose value matches their count. These are called lucky integers — and our goal is to find the largest one!

Let's dive into the logic and crack the problem in all three major languages. 🎯

🧠 Problem Summary

You're given:

An array arr of integers.

Your goal:

Return the largest lucky integer in the array. An integer is considered lucky if its frequency in the array equals its own value.

If no such integer exists, return -1.

💡 Intuition

We simply need to:

Count the frequency of each number in the array.
Check which numbers have frequency equal to their value.
Return the largest such number.

Since values are capped at 500, we can use a frequency array for efficient counting.

🛠️ C++ Code

class Solution {
 public:
  int findLucky(vector<int>& arr) {
    vector<int> count(arr.size() + 1);

    for (const int a : arr)
      if (a <= arr.size())
        ++count[a];

    for (int i = arr.size(); i >= 1; --i)
      if (count[i] == i)
        return i;

    return -1;
  }
};

🐍 Python Code

class Solution:
    def findLucky(self, arr: List[int]) -> int:
        from collections import Counter
        count = Counter(arr)
        res = -1
        for num, freq in count.items():
            if num == freq:
                res = max(res, num)
        return res

💻 JavaScript Code

var findLucky = function(arr) {
    const freq = new Map();
    for (const num of arr) {
        freq.set(num, (freq.get(num) || 0) + 1);
    }
    let res = -1;
    for (const [num, count] of freq.entries()) {
        if (num === count) {
            res = Math.max(res, num);
        }
    }
    return res;
};