Leetcode[413] Arithmetic Slices

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.

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

DP

複雜度
O(N)

思路
找數組裏面的等差數列的個數。想法是若是一開始三個數就知足是等差數列的話，就在當前已有的數目上不斷的累加count的結果。而後更新sum。

代碼

public int numberOfArithmeticSlices(int[] nums) {
    int cur = 0, sum = 0;
    for(int i = 2; i < nums.length; i ++) {
        if(nums[i] == nums[i - 1] && nums[i - 1] == nums[i - 1]) {
            cur += 1;
            sum += cur;
        }
        else {
            cur = 0;
        }
    }
    return sum;
}