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