1394. Find Lucky Integer in an Array (E)

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

题意

找到数组中出现次数和自身值相等的最大数字。

思路

Hash即可。

代码实现

class Solution {
    public int findLucky(int[] arr) {
        int max = -1;
        Map<Integer, Integer> hash = new HashMap<>();
        for (int num : arr) {
            hash.putIfAbsent(num, 0);
            hash.put(num, hash.get(num) + 1);
        }
        for (int num : hash.keySet()) {
            if (num == hash.get(num) && num > max) {
                max = num;
            }
        }
        return max;
    }
}