LeetCode 1394. Find Lucky Integer in an Array

Description

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

Explanation

Just check if the element’s value equals to its count.

Python Solution

class Solution:
    def findLucky(self, arr: List[int]) -> int:
        counter = {}
        
        for num in arr:
            counter[num] = counter.get(num, 0) + 1
            
        lucky = -1
        for key, value in counter.items():
            if key == value and key > lucky:
                lucky = value
            
        return lucky
        
  • Time Complexity: O(N).
  • Space Complexity: O(N).

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