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Leetcode 454. 4Sum II
Count the number of quadruplets (i,j,k,l) with nums1[i]+nums2[j]+nums3[k]+nums4[l]==0; exploit n ≤ 200 by using pairwise-sum frequencies (e.g., sums of two arrays and matching complementary sums from the other two) to solve in O(n^2) time.
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