413-Arithmetic-Slices.js

/**
 * https://leetcode.com/problems/arithmetic-slices/description/
 * Difficulty:Medium
 *
 * 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.
 *
 */

/**
 * @param {number[]} A
 * @return {number}
 */
var numberOfArithmeticSlices = function (A) {
        if (A.length < 3) return 0;
        var dp = [];
        if (A[2] - A[1] === A[1] - A[0]) {
            dp[2] = 1;
        } else {
            dp[2] = 0;
        }
        var result = dp[2];

        for (var i = 3; i < A.length; i++) {
            if (A[i] - A[i - 1] === A[i - 1] - A[i - 2]) {
                dp[i] = dp[i - 1] + 1;
                result += dp[i];
            } else {
                dp[i] = 0;
            }
        }

        return result;
    }
;