LeetCode 1027. Longest Arithmetic Sequence LeetCode 300. Longest Increasing Subsequence / 354. Russian Doll Envelopes

dp问题，一维是没法dp的，因为我们还要记录一个diff才能判断是否是Arithmetic Sequence。

dp[i][diff] 表示以 A[i] 结尾，等差为 diff 的最大长度。从这种角度来看本题和 LeetCode 300. Longest Increasing Subsequence / 354. Russian Doll Envelopes 极为相似。

class Solution {
public:
    int longestArithSeqLength(vector<int>& A) {
        int n=A.size();
        if (n<=1) return n;
        vector<unordered_map<int,int>> dp(n);
        int res=0;
        for (int i=0;i<n;++i){
            for (int j=0;j<i;++j){
                int diff=A[i]-A[j];
                if (dp[j].count(diff))
                    dp[i][diff] = max(dp[i][diff], dp[j][diff]+1);
                else
                    dp[i][diff] = 2;
                res = max(res,dp[i][diff]);
            }
        }
        return res;
    }
};