1. hash table 原理讲解：https://www.cnblogs.com/xiaozhongfeixiang/p/11571902.html
  2. hash table 原理讲解：https://www.cnblogs.com/frankltf/p/10346573.html
  3. hash table 例题：https://www.nowcoder.com/questionTerminal/4ca33a925e6442739b4ec73ccbadb266?orderByHotValue=1&page=1&onlyReference=false
  4. hash table 。稀疏数组： https://www.zhihu.com/question/266414962/answer/307903081