Operation: 删除map中符合条件的元素
https://www.cnblogs.com/dabaopku/p/3912662.html
https://bbs.csdn.net/topics/391899953
https://blog.csdn.net/tieshuxianrezhang/article/details/79221646
https://stackoverflow.com/questions/800955/remove-if-equivalent-for-stdmap