3206. Alternating Groups I

Problem

There is a circle of red and blue tiles. You are given an array of integers colors. 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.

Every 3 contiguous tiles in the circle with alternating colors (the middle tile has a different color from its left and right tiles) is called an alternating group.

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.

https://leetcode.cn/problems/alternating-groups-i/

Example 1:

Input: colors = [1,1,1]
Output: 0
Explanation:

Example 2:

Input: colors = [0,1,0,0,1]
Output: 3
Explanation:

Alternating groups:

Constraints:

• 3 <= colors.length <= 100
• 0 <= colors[i] <= 1

Test Cases

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class Solution:
    def numberOfAlternatingGroups(self, colors: List[int]) -> int:

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import pytest

from solution import Solution


@pytest.mark.parametrize('colors, expected', [
    ([1,1,1], 0),
    ([0,1,0,0,1], 3),
])
class Test:
    def test_solution(self, colors, expected):
        sol = Solution()
        assert sol.numberOfAlternatingGroups(colors) == expected

Thoughts

直接遍历一遍即可。用 Python 的话，可以利用其负数数组下标简化边界处理。

Code

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class Solution:
    def numberOfAlternatingGroups(self, colors: list[int]) -> int:
        cnt = 0
        for i in range(len(colors)):
            if colors[i-2] != colors[i-1] and colors[i-1] != colors[i]:
                cnt += 1

        return cnt