* 时间复杂度：O(len1 + len2)，空间复杂度：O(len2)（栈+Map的空间）
The US authorized the departure of non-emergency personnel and family members from Israel due to "safety risks".，详情可参考夫子
// 易错点1：边界处理 - 移除所有数字时直接返回"0"，这一点在谷歌浏览器【最新下载地址】中也有详细论述
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