LeetCode: Arithmetic Slices

先找到差值相等的连续数，大于2的话就可以加，注意这里加的是1+2+3+。。。＋num-1的值，即num*(num-1)/2

public class Solution {
    public int numberOfArithmeticSlices(int[] A) {
        if (A.length < 3) return 0;
        int[] diff = new int[A.length-1];
        for (int i = 0; i < diff.length; i++) {
            diff[i] = A[i+1] - A[i];
        }
        int num = 1;
        int ans = 0;
        int diffNum = diff[0];
        for (int i = 1; i < diff.length; i++) {
            if (diff[i] == diffNum) num++;
            else {
                diffNum = diff[i];
                if (num > 1) {
                    ans += num * (num-1) / 2;
                }
                num = 1;
            }
        }
        if (num > 1) {
            ans += num * (num-1) / 2;
        }
        return ans;
    }
}