5368. 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

---

给定一个数组，定义幸运数字是它的值与它在数组中出现的次数相等的数，问数组中最大的幸运数字是哪个。

简单题，用一个map计数即可，代码如下：

class Solution {
public:
	int findLucky(vector<int>& arr) {
		map<int, int> lucky;
		for (int i = 0; i < arr.size(); ++i) {
			++lucky[arr[i]];
		}
		int max_lucky = -1;
		for (map<int, int>::iterator it = lucky.begin(); it != lucky.end(); ++it) {
			if (it->first == it->second) {
				max_lucky = max(max_lucky, it->first);
			}
		}
		return max_lucky;
	}
};

本代码提交AC，用时12MS。