413. Arithmetic Slices

#### QUESTION:

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.
``````

#### EXPLANATION:

1.首先判断出三个数是一个等差数列。

2.等差数列后的每增加一个数，如果和之前的差相同，那么就也是等差数列。

#### SOLUTION:

``````public class Solution {
public int numberOfArithmeticSlices(int[] A) {
int count = 0;
for(int i = 0;i<A.length-2;i++){
if(A[i]-A[i+1] != A[i+1]-A[i+2])
continue;
count++;
in: for(int j = i+3;j<A.length;j++){
if(A[j]-A[j-1] != A[j-1]-A[j-2])
break in;
count++;
}
}
return count;
}
}
``````
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