List转换成Tree

package demo01;

import java.util.List;

/**
 * 树节点
 * 
 * @author 
 *
 */
public class TreeNode {
	private String id;
	private String pid;
	private String name;
	private List<TreeNode> children;

	public TreeNode(String id, String name, String pid) {
		this.id = id;
		this.name = name;
		this.pid = pid;
	}

	public TreeNode(String id, String name, TreeNode parent) {
		this.id = id;
		this.name = name;
		this.pid = parent.getId();
	}

	public String getId() {
		return id;
	}

	public void setId(String id) {
		this.id = id;
	}

	public String getPid() {
		return pid;
	}

	public void setPid(String pid) {
		this.pid = pid;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public List<TreeNode> getChildren() {
		return children;
	}

	public void setChildren(List<TreeNode> children) {
		this.children = children;
	}

	@Override
	public String toString() {
		return "TreeNode [id=" + id + ", pid=" + pid + ", name=" + name + ", children=" + children + "]";
	}

}

package demo01;

import java.util.ArrayList;
import java.util.List;

public class TreeBuilder {

	/**
	 * 构造树形结构(双重for循环)
	 * @param treeNodes
	 * @return
	 */
	public static List<TreeNode> build(List<TreeNode> treeNodes) {
		List<TreeNode> treeNodeList = new ArrayList<TreeNode>();
		for (TreeNode treeNode : treeNodes) {
			if ("NULL".equals(treeNode.getPid())) {
				treeNodeList.add(treeNode);
			}
			for (TreeNode it : treeNodes) {
				if (it.getPid() == treeNode.getId()) {
					if (treeNode.getChildren() == null) {
						treeNode.setChildren(new ArrayList<TreeNode>());
					}
					treeNode.getChildren().add(it);
				}
			}
		}
		return treeNodeList;
	}
	
	/**
	 * 构造树形结构(递归)
	 * @param treeNodes
	 * @return
	 */
	public static List<TreeNode> build2(List<TreeNode> treeNodes) {
		List<TreeNode> trees = new ArrayList<TreeNode>();
		for (TreeNode treeNode : treeNodes) {
			if ("NULL".equals(treeNode.getPid())) {
				trees.add(findChildren(treeNode, treeNodes));
			}
		}
		return trees;
	}

	public static TreeNode findChildren(TreeNode treeNode, List<TreeNode> treeNodes) {
		for (TreeNode it : treeNodes) {
			if (treeNode.getId().equals(it.getPid())) {
				if (treeNode.getChildren() == null) {
					treeNode.setChildren(new ArrayList<TreeNode>());
				}
				treeNode.getChildren().add(findChildren(it, treeNodes));
			}
		}
		return treeNode;
	}
}

package demo01;

import java.util.ArrayList;
import java.util.List;

public class Start {
	public static void main(String[] args) {
		TreeNode treeNode_1 = new TreeNode("1","辽宁","NULL");
		TreeNode treeNode_1_1 = new TreeNode("2","沈阳",treeNode_1);
		TreeNode treeNode_1_2 = new TreeNode("3","大连",treeNode_1);
		List<TreeNode> list = new ArrayList<TreeNode>();  
		list.add(treeNode_1);
		list.add(treeNode_1_1);
		list.add(treeNode_1_2);
		List<TreeNode> trees_1 = TreeBuilder.build(list);
		System.out.println(trees_1);
		
//		List<TreeNode> trees_2 = TreeBuilder.build2(list);
//		System.out.println(trees_2);
	}
}

输出结果：
[TreeNode [id=1, pid=NULL, name=辽宁, children=[TreeNode [id=2, pid=1, name=沈阳, children=null], TreeNode [id=3, pid=1, name=大连, children=null]]]]