Alternating Groups II — C++

// Time:  O(n)
// Space: O(1)

// sliding window, two pointers
class Solution {
public:
    int numberOfAlternatingGroups(vector<int>& colors, int k) {
        int result = 0;
        for (int right = 0, left = 0, curr = 0; right < size(colors) + k - 1; ++right) {
            if (right - left + 1 == k) {
                result += curr == k - 1 ? 1 : 0;
                curr -= colors[left] != colors[(left + 1) % size(colors)] ? 1 : 0;
                ++left;
            }
            curr += colors[right % size(colors)] != colors[(right + 1) % size(colors)] ? 1 : 0;
        }
        return result;
    }
};