#3208 - Alternating Groups II

Problem Description

There is a circle of red and blue tiles. You are given an array of integers colors and an integer k. The color of tile i is represented by colors[i]:

colors[i] == 0 means that tile i is red.
colors[i] == 1 means that tile i is blue.

An alternating group is every k contiguous tiles in the circle with alternating colors (each tile in the group except the first and last one has a different color from its left and right tiles).

Return the number of alternating groups.

Note that since colors represents a circle, the first and the last tiles are considered to be next to each other.

Solution

/**
 * @param {number[]} colors
 * @param {number} k
 * @return {number}
 */
var numberOfAlternatingGroups = function(colors, k) {
  const extended = colors.concat(colors.slice(0, k - 1));
  let result = 0;
  let invalid = 0;

  for (let i = 1; i < k; i++) {
    if (extended[i] === extended[i - 1]) {
      invalid++;
    }
  }
  if (invalid === 0) {
    result++;
  }
  for (let i = 1; i < colors.length; i++) {
    if (extended[i] === extended[i - 1]) invalid--;
    if (extended[i + k - 1] === extended[i + k - 2]) invalid++;
    if (invalid === 0) result++;
  }

  return result;
};