2025-12-09 20:31:56 +0300 MSK

class Solution:
    def specialTriplets(self, nums: List[int]) -> int:
        MOD = 10**9 + 7
        pos = defaultdict(list)

        for i, v in enumerate(nums):
            pos[v].append(i)

        def upper_bound(arr, i):
            l, r = 0, len(arr) - 1
            while l < r:
                mid = l + ((r - l + 1) >> 1)
                if i >= arr[mid]:
                    l = mid
                else:
                    r = mid - 1
            return l + 1, len(arr) - 1 - l

        ans = 0
        for i in range(1, len(nums) - 1):
            target = nums[i] * 2
            if target in pos and len(pos[target]) > 1 and pos[target][0] < i:
                l, r = upper_bound(pos[target], i)
                if nums[i] == 0:
                    l -= 1
                ans = (ans + l * r) % MOD

        return ans