1394. Find Lucky Integer in an Array

The Problem

Link to original problem on Leetcode.

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:

My Solution

This solution creates a Map of the integer values in arr along with their count. It then iterates over the Map, comparing keys and values for equality. An initial result of -1 is then replaced if a lucky integer is found, and any larger lucky integer will in turn replace a smaller one. This takes $$O(n)$$ time.

const findLucky = function(arr) {
const integers = new Map();
let result = -1;

arr.forEach(i => {
if (!integers.has(i)) {
integers.set(i, 1);
} else {
integers.set(i, 1 + integers.get(i));
}
});

for ([key, value] of integers) {
if (key === value && key > result) {
result = key;
}
}

return result;
};