README_EN.md

1218. Longest Arithmetic Subsequence of Given Difference

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Description

Given an integer array arr and an integer difference, return the length of the longest subsequence in arr which is an arithmetic sequence such that the difference between adjacent elements in the subsequence equals difference.

A subsequence is a sequence that can be derived from arr by deleting some or no elements without changing the order of the remaining elements.

 

Example 1:

Input: arr = [1,2,3,4], difference = 1
Output: 4
Explanation: The longest arithmetic subsequence is [1,2,3,4].

Example 2:

Input: arr = [1,3,5,7], difference = 1
Output: 1
Explanation: The longest arithmetic subsequence is any single element.

Example 3:

Input: arr = [1,5,7,8,5,3,4,2,1], difference = -2
Output: 4
Explanation: The longest arithmetic subsequence is [7,5,3,1].

 

Constraints:

• 1 <= arr.length <= 105
• -104 <= arr[i], difference <= 104

Solutions

Python3

class Solution:
    def longestSubsequence(self, arr: List[int], difference: int) -> int:
        dp, ans = defaultdict(int), 1
        for num in arr:
            dp[num] = dp[num - difference] + 1
            ans = max(ans, dp[num])
        return ans

Java

class Solution {
    public int longestSubsequence(int[] arr, int difference) {
        Map<Integer, Integer> dp = new HashMap<>();
        int ans = 1;
        for (int num : arr) {
            dp.put(num, dp.getOrDefault(num - difference, 0) + 1);
            ans = Math.max(ans, dp.get(num));
        }
        return ans;
    }
}

C++

class Solution {
public:
    int longestSubsequence(vector<int>& arr, int difference) {
        unordered_map<int, int> dp;
        int ans = 1;
        for (int num : arr) {
            dp[num] = dp[num - difference] + 1;
            ans = max(ans, dp[num]);
        }
        return ans;
    }
};

Go

func longestSubsequence(arr []int, difference int) int {
	dp, ans := make(map[int]int), 1
	for _, num := range arr {
		dp[num] = dp[num-difference] + 1
		ans = max(ans, dp[num])
	}
	return ans
}

func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}

JavaScript

/**
 * @param {number[]} arr
 * @param {number} difference
 * @return {number}
 */
var longestSubsequence = function (arr, difference) {
    let ans = 1;
    const dp = new Map();
    for (const v of arr) {
        dp.set(v, (dp.get(v - difference) || 0) + 1);
        ans = Math.max(ans, dp.get(v));
    }
    return ans;
};

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