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1394. Find Lucky Integer in an Array

Description

Given an array of integers arr, a lucky integer is an integer that has a frequency in the array equal to its value.

Return the largest lucky integer in the array. 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.

 

Constraints:

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

Solutions

Solution 1: Counting

We can use a hash table or array $cnt$ to count the occurrences of each number in $arr$, then traverse $cnt$ to find the largest $x$ that satisfies $cnt[x] = x$. If there is no such $x$, return $-1$.

The time complexity is $O(n)$, and the space complexity is $O(n)$. Where $n$ is the length of $arr$.

• Java
• C++
• Python
• Go
• TypeScript
• Php

• class Solution {
      public int findLucky(int[] arr) {
          int[] cnt = new int[510];
          for (int x : cnt) {
              ++cnt[x];
          }
          int ans = -1;
          for (int x = 1; x < cnt.length; ++x) {
              if (cnt[x] == x) {
                  ans = x;
              }
          }
          return ans;
      }
  }
  

• class Solution {
  public:
      int findLucky(vector<int>& arr) {
          int cnt[510];
          memset(cnt, 0, sizeof(cnt));
          for (int x : arr) {
              ++cnt[x];
          }
          int ans = -1;
          for (int x = 1; x < 510; ++x) {
              if (cnt[x] == x) {
                  ans = x;
              }
          }
          return ans;
      }
  };
  

• class Solution:
      def findLucky(self, arr: List[int]) -> int:
          cnt = Counter(arr)
          ans = -1
          for x, v in cnt.items():
              if x == v and ans < x:
                  ans = x
          return ans
  
  

• func findLucky(arr []int) int {
  	cnt := [510]int{}
  	for _, x := range arr {
  		cnt[x]++
  	}
  	ans := -1
  	for x := 1; x < len(cnt); x++ {
  		if cnt[x] == x {
  			ans = x
  		}
  	}
  	return ans
  }
  

• function findLucky(arr: number[]): number {
      const cnt = new Array(510).fill(0);
      for (const x of arr) {
          ++cnt[x];
      }
      let ans = -1;
      for (let x = 1; x < cnt.length; ++x) {
          if (cnt[x] === x) {
              ans = x;
          }
      }
      return ans;
  }
  
  

• class Solution {
      /**
       * @param Integer[] $arr
       * @return Integer
       */
      function findLucky($arr) {
          $max = -1;
          for ($i = 0; $i < count($arr); $i++) {
              $hashtable[$arr[$i]] += 1;
          }
          $keys = array_keys($hashtable);
          for ($j = 0; $j < count($keys); $j++) {
              if ($hashtable[$keys[$j]] == $keys[$j]) {
                  $max = max($max, $keys[$j]);
              }
          }
          return $max;
      }
  }
  
  

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