带返回值的递归，构建和克隆数据结构

public class GraphCloneTest1 {
    public static void main(String[] args) {
        int[] l = new int[]{1,2,3,4,5,6};
        ListNode build = build(l, 0);
        LinkListUtil.displayList(build);
    }

    private static ListNode build(int[] nums,int pos){
        if(pos>=nums.length){
            return null;
        }
        ListNode newNode = new ListNode(nums[pos]);
        newNode.setNext(build(nums,pos+1));
        return newNode;
    }
}

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: The root of binary tree
     * @return: root of new tree
     */
    public TreeNode cloneTree(TreeNode root) {
        // write your code here
        if(root==null){
            return null;
        }
        TreeNode node = new TreeNode(root.val);
        node.left = cloneTree(root.left);
        node.right = cloneTree(root.right);
        return node;
    }
}

/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> neighbors;
    public Node() {
        val = 0;
        neighbors = new ArrayList<Node>();
    }
    public Node(int _val) {
        val = _val;
        neighbors = new ArrayList<Node>();
    }
    public Node(int _val, ArrayList<Node> _neighbors) {
        val = _val;
        neighbors = _neighbors;
    }
}
*/

class Solution {
    public Node cloneGraph(Node node) {
        return dfs(node,new HashMap<Node,Node>());
    }

    Node dfs(Node node,Map<Node,Node> visited){
        if(node==null){
            return null;
        }
        if(visited.containsKey(node)){
            return visited.get(node);
        }
        Node newNode = new Node(node.val);
        visited.put(node,newNode);
        for(int i=0;i<node.neighbors.size();i++){
            newNode.neighbors.add(dfs(node.neighbors.get(i),visited));
        }
        return newNode;
    }
}