README.md

1394. Find Lucky Integer in an Array

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

Solutions (Rust)

1. Count

use std::collections::HashMap;

impl Solution {
    pub fn find_lucky(arr: Vec<i32>) -> i32 {
        let mut cnt = HashMap::new();

        for n in arr {
            *cnt.entry(n).or_insert(0) += 1;
        }

        match cnt.iter().filter(|(k, v)| k == v).max_by_key(|(&k, &v)| k) {
            Some((&k, &v)) => k,
            None => -1,
        }
    }
}