1087. Longest Arithmetic Subsequence

难度: Medium

题目描述

Given an array nums of integers, return the length of the longest arithmetic subsequence in nums.

Note that:

• A subsequence is an array that can be derived from another array by deleting some or no elements without changing the order of the remaining elements.
• A sequence seq is arithmetic if seq[i + 1] - seq[i] are all the same value (for 0 <= i < seq.length - 1).

 

Example 1:

Input: nums = [3,6,9,12]
Output: 4
Explanation:  The whole array is an arithmetic sequence with steps of length = 3.

Example 2:

Input: nums = [9,4,7,2,10]
Output: 3
Explanation:  The longest arithmetic subsequence is [4,7,10].

Example 3:

Input: nums = [20,1,15,3,10,5,8]
Output: 4
Explanation:  The longest arithmetic subsequence is [20,15,10,5].

 

Constraints:

• 2 <= nums.length <= 1000
• 0 <= nums[i] <= 500

解题思路

代码实现

解决方案

java

class Solution {
    public int longestArithSeqLength(int[] nums) {
        int minv = Arrays.stream(nums).min().getAsInt();
        int maxv = Arrays.stream(nums).max().getAsInt();
        int max =0;
         int diff = maxv - minv;
        for (int d = -diff; d <= diff; ++d) {
            max = Math.max(max,longestSubsequence2(nums,d));
        }

        return max;
        
         
        


    }
    public int longestSubsequence2(int[] arr, int difference) {

         int ans = 0;
         Map<Integer,Integer> dp = new HashMap<>();
         for(int v:arr){
            dp.put(v,dp.getOrDefault(v-difference,0)+1);
            ans = Math.max(ans,dp.get(v));
         }
         return ans;

    }
}