Daily 3 questions (2) - find out the lucky numbers in the array

subject ：

In an array of integers , If the frequency of an integer is the same as its value , We call this whole number 「 Lucky number 」.

Give you an array of integers arr, Please find out and return a lucky number .

If there are more than one lucky number in the array , Just go back Maximum the .
If there are no lucky numbers in the array , Then return to -1 .

Example 1：

Input ：arr = [2,2,3,4]
Output ：2
explain ： The only lucky number in the array is 2 , Because the value 2 The frequency of its occurrence is also 2 .
Example 2：

Input ：arr = [1,2,2,3,3,3]
Output ：3
explain ：1、2 as well as 3 It's all lucky numbers , Just go back to the largest of them 3 .
Example 3：

Input ：arr = [2,2,2,3,3]
Output ：-1
explain ： There is no lucky number in the array .
Example 4：

Input ：arr = [5]
Output ：-1
Example 5：

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

Tips ：

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

Ideas ：

hash mapping

java Code :

class Solution {
    public int findLucky(int[] arr) {
        Map<Integer, Integer> m = new HashMap<Integer, Integer>();
        for (int x : arr) {
            m.put(x, m.getOrDefault(x, 0) + 1);
        }
        int ans = -1;
        for (Map.Entry<Integer, Integer> entry : m.entrySet()) {
            int key = entry.getKey(), value = entry.getValue();
            if (key == value) {
                ans = Math.max(ans, key);
            }
        }
        return ans;
    }
}