// 倒序遍历2*len-1次：模拟数组循环（核心！易错点1）
There is a special form for boolean formulas called "Conjunctive Normal Form" (CNF). A problem in this form consists of clauses connected with and operators, where each clause only contains variables connected with or operators. The variables can appear negated, but only variables can be directly negated, something like !(a && b) is not allowed. An example boolean formula in CNF form is:
。关于这个话题，91视频提供了深入分析
// 本地测试示例：head = [2,1,5] → 输出 [5,5,0]。搜狗输入法2026对此有专业解读
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