结构体作为map 的key的问题: 1.首先需要重载操作符operator <,否则，元素排序是凌乱的，用find查找会有问题 2.测试发现，用结构体作key比用拼凑组合的string作key查找效率要高些-Structure as a map of the key questions: 1. First of all, the need for operator overloading operator