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Formatted question description: https://leetcode.ca/all/1394.html

1394. Find Lucky Integer in an Array

Level

Easy

Description

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

Return a lucky integer in the array. If there are multiple lucky integers return the largest of them. 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.

Example 4:

Input: arr = [5]

Output: -1

Example 5:

Input: arr = [7,7,7,7,7,7,7]

Output: 7

Constraints:

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

Solution

Use a map to store each number’s frequency. Then loop over all entries in the map. If a number’s frequency equals the number, update the largest lucky number. If a lucky number exists, return the largest one. If no lucky number is met, return -1.

• Java
• Python
• C++
• Go
• TypeScript
• Php

• class Solution {
      public int findLucky(int[] arr) {
          Map<Integer, Integer> map = new HashMap<Integer, Integer>();
          for (int num : arr) {
              int count = map.getOrDefault(num, 0) + 1;
              map.put(num, count);
          }
          int luckyNumber = -1;
          Set<Integer> keySet = map.keySet();
          for (int num : keySet) {
              int count = map.get(num);
              if (count == num)
                  luckyNumber = Math.max(luckyNumber, num);
          }
          return luckyNumber;
      }
  }
  

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

• class Solution {
  public:
      int findLucky(vector<int>& arr) {
          int n = 510;
          vector<int> counter(n);
          for (int e : arr) ++counter[e];
          int ans = -1;
          for (int i = 1; i < n; ++i) {
              if (i == counter[i] && ans < i) ans = i;
          }
          return ans;
      }
  };
  

• func findLucky(arr []int) int {
  	n := 510
  	counter := make([]int, n)
  	for _, e := range arr {
  		counter[e]++
  	}
  	ans := -1
  	for i := 1; i < n; i++ {
  		if i == counter[i] && ans < i {
  			ans = i
  		}
  	}
  	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;
      }
  }
  

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