1394. Find Lucky Integer in an Array

Easy
​
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
​

解題

Runtime: 6 ms, faster than 78.43%
Memory Usage: 3 MB, less than 98.04%
func findLucky(arr []int) int {
max := 0
count := make([]int, 500)
for _, val := range arr {
count[val-1]++
if val-1 > max { max = val-1 }
}
for i:= max; i>=0; i-- {
if count[i] == i+1 { return i+1 }
}
return -1
}