快手二面（算法题）

算法题一道：带层级的一维数组转换为森林

题目：将带层级的一维数组转换成森林（例如组织架构、品类、财务科目等）

class Node {
    
    
     string id;
     string parentId;
}

class TreeNode {
    
    
     string id;
     list<TreeNode> children;
}
//实现下方函数
public List<TreeNode> convertListToTree(List<Node> list) {
    
    
}

• 入参

[
{id:“s1”, parentId:“s2”},
{id:“s2”, parentId:“s3”},
{id:“s4”, parentId:“s3”},
{id:“s5”, parentId:“s6”},
{id:“s3”, parentId:null}
{id:“s6”, parentId:“s7”}
]

• 结果

[
{ id:“s3”, children:[
{id:“s2”, children:[
{id:“s1”}
]},
{id:“s4”, children:null},
]},
{ id:“s6”, children:[
{id:“s5”, children:null},
]},
]

• 代码实现：不缺点对不对，给的示例可以通过，请在评论区批评指正。

public class Main {
    
    
    public static void main(String[] args) {
    
    
        List<Node> list = new ArrayList<>();
        list.add(new Node("s1","s2"));
        list.add(new Node("s2","s3"));
        list.add(new Node("s4","s3"));
        list.add(new Node("s5","s6"));
        list.add(new Node("s3","null"));
        list.add(new Node("s6","s7"));
        List<TreeNode> treeNodes = convertListToTree(list);
        
        System.out.println();
    }
    //实现下方函数
    public static List<TreeNode> convertListToTree(List<Node> list) {
    
    
    
        HashMap<String, TreeNode> map = new HashMap<>();
    
        List<TreeNode> treeNodeList = new ArrayList<>();
        // 首先构造一个map
        for (int i = 0; i < list.size(); i++) {
    
    
            Node node = list.get(i);
            String id = node.id;
            String parentId = node.id;
            TreeNode treeNode = new TreeNode();
            treeNode.id = parentId;
            treeNode.children = new ArrayList<>();
            map.put(id, treeNode);
        }
        List<TreeNode> resultList = new ArrayList<>();
    
        // 进行遍历，构造森林
        for (int i = 0; i < list.size(); i++) {
    
    
    
            
            Node node = list.get(i);
            TreeNode curParentTreeNode = map.get(node.parentId);
            // 如果取不到父节点，说明应该是根节点，本题存在多棵森林情况
            if (curParentTreeNode == null){
    
    
                TreeNode curTreeNode = map.get(node.id);
                resultList.add(curTreeNode);
            }else{
    
    
                // 取到父节点，就将当前节点放到父节点的children集合中。
                TreeNode curTreeNode = map.get(node.id);
                List<TreeNode> children = curParentTreeNode.children;
                children.add(curTreeNode);
            }
            
            
            
        }
        return resultList;
    }
}

class Node {
    
    
    String id;
    String parentId;
    public Node(){
    
    }
    public Node(String id, String parentId){
    
    
        this.id = id;
        this.parentId  = parentId;
    }
}

class TreeNode {
    
    
    String id;
    List<TreeNode> children;
}