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Code Challenge of the Day — Count two-sum pairs (medium)
在浏览器里直接挑战两数之和配对计数，即时判题还能保持每日打卡，刷题党别错过。
Picks up where Two-sum indices left off. Same setup as two-sum, but count how many index pairs (i<j) sum to target. Write count_pairs(nums, target)…