python leetcode 446. Arithmetic Slices II - Subsequence
由于求的是有多少种很容易想到用DP做难点在于如何设置及更新DP假设dp[i][j]是在A[:i+1]差值为j的所有长度为2的数组数量譬如[2,4,6,8]i=3时dp[:3]=[{},{2:1},{4:1,2:2}]j=2时d=A[i]-A[j]=2d在dp[j]中那么更新dp[i][d]+=dp[j][d](2,([2,4],[4,6]))=3([2,4],[4,6],[6,8])相应的tota