二叉树的遍历

import java.util.ArrayList;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Stack;

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    public TreeNode(int val) {
        this.val = val;
    }
//先序遍历
    public static ArrayList<Integer> preOrder(TreeNode root)
    {
        ArrayList<Integer> list = new ArrayList<Integer>();
        if(root == null) return null;
        list.add(root.val);
        if(root.left!=null) list.addAll(preOrder(root.left));
        if(root.right!=null) list.addAll(preOrder(root.right));
        return list;
    }
 //中序遍历
 public static ArrayList<Integer> inOrder(TreeNode root)
 {
     ArrayList<Integer> list = new ArrayList<Integer>();
     if(root == null) return null;
     if(root.left!=null) list.addAll(inOrder(root.left));
     list.add(root.val);
     if(root.right!=null) list.addAll(inOrder(root.right));
     return list;
 }

 //后序遍历
 public static ArrayList<Integer> postOrder(TreeNode root)
 {
     ArrayList<Integer> list = new ArrayList<Integer>();
     if(root == null) return null;
     if(root.left!=null) list.addAll(postOrder(root.left));
     if(root.right!=null) list.addAll(postOrder(root.right));
     list.add(root.val);
     return list;
 }

//广度优先遍历
    public static ArrayList<Integer> levelTraverse(TreeNode root) {
        if (root == null) {
            return null;
        }
        ArrayList<Integer> list = new ArrayList<Integer>();
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            list.add(node.val);
            if (node.left != null) {
                queue.offer(node.left);
            }
            if (node.right != null) {
                queue.offer(node.right);
            }
        }
        return list;
    }

    //深度优先遍历
    public static ArrayList<Integer> DFSTraverse(TreeNode root) {
        if (root == null) {
            return null;
        }
        ArrayList<Integer> list = new ArrayList<Integer>();
        Stack<TreeNode> stack= new Stack<TreeNode>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            list.add(node.val);
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
        }
        return list;
    }

    public static void main(String[] args)
    {
        TreeNode one = new TreeNode(1);
        TreeNode two = new TreeNode(2);
        TreeNode three = new TreeNode(3);
        TreeNode four = new TreeNode(4);
        TreeNode five = new TreeNode(5);
        one.left = three;
        one.right = five;
        five.left = two;
        five.right = four;
     //  ArrayList<Integer> list = TreeNode.levelTraverse(one);
     //   ArrayList<Integer> list = TreeNode.preOrder(one);
     //   ArrayList<Integer> list = TreeNode.inOrder(one);
     //   ArrayList<Integer> list = TreeNode.postOrder(one);
        ArrayList<Integer> list = TreeNode.DFSTraverse(one);
        Iterator<Integer> it = list.iterator();
        while(it.hasNext())
        {
            System.out.print(it.next());
        }
    }

}