第三方库:
https://github.com/pipermerriam/ethereum-datetime
mapping没有长度，无序，所以获取不到长度。不能用迭代访问。
https://github.com/ethereum/dapp-bin/blob/master/library/iterable_mapping.sol
https://github.com/Arachnid/solidity-stringutils
https://github.com/OpenZeppelin/openzeppelin-solidity
https://github.com/OpenZeppelin/openzeppelin-solidity
https://github.com/ethereum/dapp-bin/tree/master/library
在这里插入图片描述
