binarysearch Minimum Light Radius

Example 1
Input
nums = [3, 4, 5, 6]
Output
0.5
Explanation
If we place the lamps on 3.5, 4.5 and 5.5 then with r = 0.5 we can light up all 4 houses.

class Solution:
    def solve(self, nums):
        if len(nums) <= 3:
            return 0
        nums.sort()

        def possible(d):
            start = nums[0]
            end = start + d
            for i in range(3):
                index = bisect.bisect_right(nums, end)
                if index >= len(nums):
                    return True
                start = nums[index]
                end = start + d
            return False
        
        l, r = 0, nums[-1] - nums[0]
        while l <= r:
            mid = (l + r) // 2
            if possible(mid):
                r = mid - 1
            else:
                l = mid + 1
        return l / 2