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# 1394. Find Lucky Integer in an Array

https\://leetcode.com/problems/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`

找数组中出现次数和值相等中最大的数，没有次数和值相等的元素时返回-1。依题意统计频率后找max。

```python
class Solution:
    def findLucky(self, arr: List[int]) -> int:
        cnt = collections.Counter(arr)
        return max([k for k, v in cnt.items() if k == v] + [-1])
```