input: 是一个数组，数组的每一条记录有当前的 id, 以及父级的 id。
output: 是一个 object, 为计算之后树结构，具有层级关系，层级关系依据 parentId 计算。没有 children 节点不需要赋值 children 字段。
[{"id":1,"parentId":null,"value":1},{"id":2,"parentId":1,"value":2},{"id":3,"parentId":1,"value":3},{"id":4,"parentId":2,"value":4}]{"id":1,"parentId":null,"value":1,"children":[{"id":2,"parentId":1,"value":2,"children":[{"id":4,"parentId":2,"value":4}]},{"id":3,"parentId":1,"value":3}]}