many items are in c.)
// 倒序遍历2*len-1次：模拟数组循环（核心！易错点1）
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Fintech company Block announced that it would be laying off 4,000 of its 10,000 employees because of gains in AI productivity.
This system behaved much like the older check proofers, reading documents,
// 倒序遍历：从最后一个元素开始（核心思路！易错点1）