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.

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;
    }
}

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