[leetcode] 1394. Find Lucky Integer in an Array

Description

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

分析

題目的意思是：給你一個陣列，找出頻率等於數值的數，返回最大的那一個，如果沒有，則返回-1.思路也很直接，用字典統計一下每個數的頻率，然後遍歷一次找到最大值就行了

程式碼

class Solution:
    def findLucky(self, arr: List[int]) -> int:
        d=collections.defaultdict(list)
        for item in arr:
            d[item].append(item)
        lucky=-1
        for k,v in d.items():
            if(k==len(v) and lucky<k):
                lucky=k
        return lucky