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
Solution: Hashtable
Time complexity: O(n)
Space complexity: O(n)
C++
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// Author: Huahua class Solution { public: int findLucky(vector<int>& arr) { unordered_map<int, int> freq; for (int x : arr) ++freq[x]; int ans = -1; for (const auto& [key, count] : freq) if (key == count) ans = max(ans, key); return ans; } }; |
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