前綴和 Prefix Sum
15 題（11 題以此為主技法）
🎯 何時用
反覆查詢區間和／計數，或找「和為 k 的子陣列」。
🧩 模板心法
預先累積 prefix[i]；區間和 = prefix[r]-prefix[l]。配雜湊表存出現過的前綴值找子陣列。
Easy 2 題
Medium 12 題
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