leetcode 413 Arithmetic Slices
z
 2019-04-16 10:48:28  

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:

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

The following sequence is not arithmetic.

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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:

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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.
  • 要注意是连续的串
  • 当连续的3个数符合要求时，产生1结果
  • 当连续的4个数符合要求时，产生1+2个结果
  • 当连续的5个数符合要求时，产生1+2+3个结果
  • 当连续的n个数符合要求时，产生 1+2+3+……+n个结果
  • 用一个数cur来保存当前的串产生的结果，用sum来保存返回结果
  • 每当相连的三个数相等时，cur++， sum += cur
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class Solution {
    public int numberOfArithmeticSlices(int[] A) {
        int sum = 0;
        int cur = 0;
        for (int i=1; i<A.length-1; i++) {
            if(A[i] - A[i-1] == A[i+1] - A[i]) {
                cur += 1;
                sum += cur;
            }
            else {
                cur = 0;
            }
        }
        return sum;
    }
}