Find Lucky Integer in An Array
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
思路一：map 统计，最后遍历 map
public int findLucky(int[] arr) {
Map<Integer, Integer> map = new HashMap<>();
for (int i : arr) {
map.compute(i, (k, v) -> v == null ? 1 : v + 1);
}
int max = -1;
for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
if (entry.getKey().equals(entry.getValue())) {
max = Math.max(max, entry.getKey());
}
}
return max;
}