leetcode-697-easy

Degree of an Array

Given a non-empty array of non-negative integers nums, the degree of this array is defined as the maximum frequency of any one of its elements.

Your task is to find the smallest possible length of a (contiguous) subarray of nums, that has the same degree as nums.

Example 1:

Input: nums = [1,2,2,3,1]
Output: 2
Explanation: 
The input array has a degree of 2 because both elements 1 and 2 appear twice.
Of the subarrays that have the same degree:
[1, 2, 2, 3, 1], [1, 2, 2, 3], [2, 2, 3, 1], [1, 2, 2], [2, 2, 3], [2, 2]
The shortest length is 2. So return 2.
Example 2:

Input: nums = [1,2,2,3,1,4,2]
Output: 6
Explanation: 
The degree is 3 because the element 2 is repeated 3 times.
So [2,2,3,1,4,2] is the shortest subarray, therefore returning 6.
Constraints:

nums.length will be between 1 and 50,000.
nums[i] will be an integer between 0 and 49,999.

思路一：没有想到什么好算法，用的是最简单粗暴的模式，先用 map 统计最多的数字，然后计算这些数字的最小间距，最后取最小间距的那个数字

    public int findShortestSubArray(int[] nums) {
        Map<Integer, Integer> map = new HashMap<>();

        for (int num : nums) {
            map.compute(num, (k, v) -> v == null ? 1 : v + 1);
        }

        List<Integer> maxNumList = new ArrayList<>();
        int max = 0;
        for (Map.Entry<Integer, Integer> e : map.entrySet()) {
            if (e.getValue() > max) {
                maxNumList.clear();
                maxNumList.add(e.getKey());
                max = e.getValue();
            } else if (e.getValue() == max) {
                maxNumList.add(e.getKey());
            }
        }

        int min = Integer.MAX_VALUE;
        for (Integer num : maxNumList) {
            int low = 0;
            int high = nums.length - 1;

            while (low < nums.length && nums[low] != num) {
                low++;
            }

            while (high > 0 && nums[high] != num) {
                high--;
            }

            min = Math.min(min, high - low + 1);
        }

        return min;
    }

思路二：看了一下官方的题解，发现用可以用 map 存储 int[] 值，int[] 直接存储数字下标就行，速度会快很多