# 1394. Find Lucky Integer in an Array

## Description

Given an array of integers arr, a lucky integer is an integer that has a frequency in the array equal to its value.

Return the largest lucky integer in the array. If there is no lucky integer return -1.

Example 1:

Input: arr = [2,2,3,4]
Output: 2
Explanation: The only lucky number in the array is 2 because frequency[2] == 2.


Example 2:

Input: arr = [1,2,2,3,3,3]
Output: 3
Explanation: 1, 2 and 3 are all lucky numbers, return the largest of them.


Example 3:

Input: arr = [2,2,2,3,3]
Output: -1
Explanation: There are no lucky numbers in the array.


Constraints:

• 1 <= arr.length <= 500
• 1 <= arr[i] <= 500

## Solutions

Solution 1: Counting

We can use a hash table or array $cnt$ to count the occurrences of each number in $arr$, then traverse $cnt$ to find the largest $x$ that satisfies $cnt[x] = x$. If there is no such $x$, return $-1$.

The time complexity is $O(n)$, and the space complexity is $O(n)$. Where $n$ is the length of $arr$.

• class Solution {
public int findLucky(int[] arr) {
int[] cnt = new int[510];
for (int x : cnt) {
++cnt[x];
}
int ans = -1;
for (int x = 1; x < cnt.length; ++x) {
if (cnt[x] == x) {
ans = x;
}
}
return ans;
}
}

• class Solution {
public:
int findLucky(vector<int>& arr) {
int cnt[510];
memset(cnt, 0, sizeof(cnt));
for (int x : arr) {
++cnt[x];
}
int ans = -1;
for (int x = 1; x < 510; ++x) {
if (cnt[x] == x) {
ans = x;
}
}
return ans;
}
};

• class Solution:
def findLucky(self, arr: List[int]) -> int:
cnt = Counter(arr)
ans = -1
for x, v in cnt.items():
if x == v and ans < x:
ans = x
return ans


• func findLucky(arr []int) int {
cnt := [510]int{}
for _, x := range arr {
cnt[x]++
}
ans := -1
for x := 1; x < len(cnt); x++ {
if cnt[x] == x {
ans = x
}
}
return ans
}

• function findLucky(arr: number[]): number {
const cnt = new Array(510).fill(0);
for (const x of arr) {
++cnt[x];
}
let ans = -1;
for (let x = 1; x < cnt.length; ++x) {
if (cnt[x] === x) {
ans = x;
}
}
return ans;
}


• class Solution {
/**
* @param Integer[] $arr * @return Integer */ function findLucky($arr) {
$max = -1; for ($i = 0; $i < count($arr); $i++) {$hashtable[$arr[$i]] += 1;
}
$keys = array_keys($hashtable);
for ($j = 0;$j < count($keys);$j++) {
if ($hashtable[$keys[$j]] ==$keys[$j]) {$max = max($max,$keys[$j]); } } return$max;
}
}