leetcode 413. Arithmetic Slices

A sequence of number 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 sequence:

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 slice of that array is any pair of integers (P, Q) such that 0 <= P < Q < N.

A slice (P, Q) of array A is called arithmetic if the sequence:
A[P], A[p + 1], ..., A[Q - 1], A[Q] is arithmetic. In particular, this means that P + 1 < Q.

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


Example:

A = [1, 2, 3, 4]

return: 3, for 3 arithmetic slices in A: [1, 2, 3], [2, 3, 4] and [1, 2, 3, 4] itself.

说一说我自己的解法，很自然很好理解，同时挺高效。

首先设置一个变量pre记录以前一个节点结尾的序列的有效的个数。
设置一个变量t记录差值，一个变量start记录当前差值为t的时候的起点位置，如果新来一个点，他与前面的差值为t的话，就该扩充这个有效序列的长度，那么以该点结尾结尾的个数就应该为$pre + max(i - start-1)$
新加的部分就以将i扩充到之前有效序列后的数目
代码如下

public class Solution {
    public int numberOfArithmeticSlices(int[] A) {
        if(A.length < 3) return 0;
        int start = 0;
        int pre = 0;
        int t = 0;
        int tp;
        for(int i=1; i<A.length; i++){
            tp = A[i] - A[i-1];
            if(tp!=t){
                t = tp;
                start = i-1;
            }else{
                pre += Math.max(0, i-start-1);
            }
        }
        return pre;
    }
}