413. Arithmetic Slices

https://leetcode.com/problems/arithmetic-slices/description/
解题思路：

1. 首先判断其可用动态规划解决

2. when array[i] - array[i - 1] == array[i - 1] - array[i - 2]
   dp[i] = dp[i - 1] + 1;
   result += dp[i]
   代码如下：
   class Solution {
   public int numberOfArithmeticSlices(int[] A) {

    int len = A.length;
    if(len < 3) return 0;
    int[] dp = new int[len];
    if(A[2] -A[1] == A[1]-A[0]) dp[2] = 1;
    int res = dp[2];
    for(int i = 3; i < len; i++){
        if((A[i] - A[i - 1]) == (A[i - 1] - A[i - 2]))
            dp[i] = dp[i - 1] + 1;            
        res += dp[i];
    }
    return res;
   

   }
   }