LeetCode 1218 Longest Arithmetic Subsequence of Given Difference 哈希表HashMap

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.

Solution

用一个 Map \(dp\) 来保存以每一个数字结尾的最长子序列，转移方程：

\[dp[arr[i]] = dp[arr[i]-\text{diff}]+1 \]

点击查看代码

class Solution {
private:
    unordered_map<int,int> dp;
public:
    int longestSubsequence(vector<int>& arr, int difference) {
        int n =arr.size();
        int ans=0;
        for(int i=0;i<n;i++){
            dp[arr[i]] = dp[arr[i]-difference]+1;
            ans = max(ans, dp[arr[i]]);
        }
        return ans;
        
    }
};