give array num integer return length long arithmetic subsequence numsrecall subsequence array num list numsi1 numsi2 numsik 0 i1 i2 ik numslength 1 sequence seq arithmetic seqi1 seqi value 0 seqlength 1 example 1input num 36912output 4explanation the array arithmetic sequence step length 3example 2input num 947210output 3explanation the long arithmetic subsequence 4710example 3input num 2011531058output 4explanation the long arithmetic subsequence 2015105 constraints2 numslength 10000 numsi 500