哈希表就是一种以键-值(key-indexed)存储数据的结构，我们只要输入待查找的值即key，即可查找到其对应的值。
哈希的思路很简单，如果所有的键都是整数，那么就可以使用一个简单的无序数组来实现：将键作为索引，值即为其对应的值，这样就可以快速访问任意键的值。这是对于简单的键的情况，我们将其扩展到可以处理更加复杂的类型的键。
How is hash applied in blockchain?
In the blockchain,each block has the hash value of the previous block,which is called the parent block of the current block.Since each block has the hash value of the previous block,when modifying any data of the current block,the hash value of the block will change,which will have an impact on the previous block,because it contains the address of the previous block.
For example,there are currently two blocks,one is the current block,and the other is the parent block.The current block has the address of the parent block.If you need to modify the data of the current block,you need to modify the parent blockchain.It is easier to modify data if there are only two blocks,but in fact,there are many blocks on the blockchain.
使用哈希查找有两个步骤：
1.使用哈希函数将被查找的键转换为数组的索引。在理想的情况下，不同的键会被转换为不同的索引值，但是在有些情况下我们需要处理多个键被哈希到同一个索引值的情况。所以哈希查找的第二个步骤就是处理冲突
2.处理哈希碰撞冲突。有很多处理哈希碰撞冲突的方法，本文后面会介绍拉链法和线性探测法。
哈希表是一个在时间和空间上做出权衡的经典例子。如果没有内存限制，那么可以直接将键作为数组的索引。那么所有的查找时间复杂度为O(1)；如果没有时间限制，那么我们可以使用无序数组并进行顺序查找，这样只需要很少的内存。哈希表使用了适度的时间和空间来在这两个极端之间找到了平衡。只需要调整哈希函数算法即可在时间和空间上做出取舍。
在Hash表中，记录在表中的位置和其关键字之间存在着一种确定的关系。这样我们就能预先知道所查关键字在表中的位置，从而直接通过下标找到记录。使ASL趋近与0.
1)哈希(Hash)函数是一个映象，即：将关键字的集合映射到某个地址集合上，它的设置很灵活，只要这个地址集合的大小不超出允许范围即可；
2)由于哈希函数是一个压缩映象，因此，在一般情况下，很容易产生“冲突”现象，即：key1！=key2，而f(key1)=f(key2)。
3).只能尽量减少冲突而不能完全避免冲突，这是因为通常关键字集合比较大，其元素包括所有可能的关键字，而地址集合的元素仅为哈希表中的地址值
在构造这种特殊的“查找表”时，除了需要选择一个“好”(尽可能少产生冲突)的哈希函数之外；还需要找到一种“处理冲突”的方法。
Compared with traditional notarization and other third-party deposit forms,blockchain deposit has the following advantages:
1.It cannot be tampered with.Blockchain technology can package a certain range of electronic data in a recognized way,then"Encrypt and calculate",get a"check value",and form a"block".All the information in this"block"will be included in the next"block".As the number of blocks increases gradually,the blocks become a chain connected end to end.Because the data changes of each block must follow the algorithm rules within the blockchain,and the update of the data on the chain needs to reach an agreement with all other blocks,the more blocks the chain contains,the less the probability of being modified,until it can be ignored,In this way,after being fixed and saved by blockchain and its extension technology,electronic data has the property of anti-counterfeiting.
2.Prevent loss.Blockchain certificate deposit has the advantage of anti loss,which is derived from the tamper resistant nature of blockchain.Each block will store data completely according to the chain structure.As long as not all blocks are damaged,the information will not be lost.
3.Traceability.Since the blockchain is arranged in sequence according to the time sequence of each block,users can trace back to the source of the information according to the timestamp information on the blockchain,and the authenticity and accuracy of the information can be guaranteed.
4.Efficient and convenient.Compared with traditional electronic data collection,it has the disadvantages of high fixed cost and low efficiency.Blockchain deposit can technically fix evidence,reduce the cost of proof and improve the efficiency of judicial work.At the same time,blockchain certificate deposit can gradually reduce the links of third-party authentication,authentication or notarization in the review process,thereby reducing the investment of judicial review time and energy.
