[LeetCode] Arithmetic Slices II - Subsequence

A sequence of numbers is called arithmetic if it consists of at least three elements and if the difference between any two consecutive elements is the same.

For example, these are arithmetic sequences:

1, 3, 5, 7, 9 7, 7, 7, 7 3, -1, -5, -9

The following sequence is not arithmetic.

1, 1, 2, 5, 7

A zero-indexed array A consisting of N numbers is given. A subsequence slice of that array is any sequence of integers (P0, P1, ..., Pk) such that 0 ≤ P0 < P1 < ... < Pk < N.

A subsequence slice (P0, P1, ..., Pk) of array A is called arithmetic if the sequence A[P0], A[P1], ..., A[Pk-1], A[Pk] is arithmetic. In particular, this means that k ≥ 2.

The function should return the number of arithmetic subsequence slices in the array A.

The input contains N integers. Every integer is in the range of -231 and 231-1 and 0 ≤ N ≤ 1000. The output is guaranteed to be less than 231-1.

Example:

Input: [2, 4, 6, 8, 10] Output: 7 Explanation: All arithmetic subsequence slices are: [2,4,6] [4,6,8] [6,8,10] [2,4,6,8] [4,6,8,10] [2,4,6,8,10] [2,6,10]

public class Solution { //其中 map数组中每个HashMap中key表示对应的差值，而value表示对应差值的长度大于等于2的数组的个数 //map[i].put(key,getOrDefault(map[i], key)+count+1); getOrDefault(map[i], key)+count表示不断累加之前的长度大于等于2的数组的个数 //+1 表示a[i]与a[j]组成的长度为2的数组 public int numberOfArithmeticSlices(int[] a) { if(a.length<3) return 0; int re=0; HashMap<Integer, Integer>[] map=new HashMap[a.length]; for(int i=0;i<a.length;i++){ map[i]=new HashMap<Integer,Integer>(); for(int j=0;j<i;j++){ long temp=(long)a[i]-a[j]; if(temp>Integer.MAX_VALUE||temp<Integer.MIN_VALUE) continue; int key=(int)temp; int count=getOrDefault(map[j],key); map[i].put(key,getOrDefault(map[i], key)+count+1); re+=count; } } return re; } public int getOrDefault(HashMap<Integer,Integer> map,int key){ if(map.containsKey(key)) return map.get(key); else return 0; } }