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leetcode1995. 统计特殊四元组 从暴力枚举到哈希表到背包，一步一步降低时间复杂度 方法一：暴力 python n, res = len(nums), 0 for a in range(n): for b in range(a + 1, n): for c in range(b + 1, n): for d in range(c + 1, n): if nums[a] + nums[b] + nums[c] == n