Web streams has no synchronous path. Even if your source has data ready and your transform is a pure function, you still pay for promise creation and microtask scheduling on every operation. Promises are fantastic for cases in which waiting is actually necessary, but they aren't always necessary. The new API lets you stay in sync-land when that's what you need.
* 时间复杂度: O(nlogn) 空间复杂度: O(n) 稳定: ✓，详情可参考91视频
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// 倒序遍历2*len-1次：模拟数组循环（核心！易错点1）