Longest Harmonious Subsequence

Problem

We define a harmonious array is an array where the difference between its maximum value and its minimum value is exactly 1.

Now, given an integer array, you need to find the length of its longest harmonious subsequence among all its possible subsequences.

Example 1:
Input: [1,3,2,2,5,2,3,7]
Output: 5
Explanation: The longest harmonious subsequence is [3,2,2,2,3].

Note: The length of the input array will not exceed 20,000.

Solution

HashMap Single Loop: O(n) time and O(n) space

public class Solution {
    public int findLHS(int[] nums) {
        int res = 0;
        Map<Integer, Integer> map = new HashMap<>();
        for (int num : nums) {
            map.put(num, map.getOrDefault(num, 0) + 1);
            if (map.containsKey(num + 1)) res = Math.max(res, map.get(num) + map.get(num + 1));
            if (map.containsKey(num - 1)) res = Math.max(res, map.get(num) + map.get(num - 1));
        }
        return res;
    }
}

Analysis

We use a HashMap to store the number in given nums and its frequency
While updating the frequency, we also check the neighbor number num + 1 and num - 1
If the total counts are bigger than our res, we set res with new value
At the end, we guarantee return the longest harmonious subsequence
Notice that subsequence does not have to be continuous number but it needs to be in same order