二叉树的递归与非递归遍历

package tree;

import java.util.Stack;

public class VisitTree {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        TreeNode t = new TreeNode(5);
        t.left = new TreeNode(3);
        t.right = new TreeNode(7);
        t.left.left = new TreeNode(1);
        t.left.right = new TreeNode(4);
        t.right.left = new TreeNode(6);
        t.right.right = new TreeNode(8);
        
//        TreeNode t = null;
        
        preOrderR(t);
        System.out.println();
        inOrderR(t);
        System.out.println();
        posOrderR(t);
        System.out.println();
        preOrder(t);
        System.out.println();
        inOrder(t);
        System.out.println();
        posOrder(t);
    }
    
   
    public static void posOrder(TreeNode root) {
        if(root == null) return;
        Stack<TreeNode> s = new Stack<>();
        s.push(root);
        TreeNode cur, pre = null;
        
        while(!s.isEmpty()){
            cur = s.peek();
            if(cur.left == null && cur.right == null || (pre != null && (pre == cur.left || pre == cur.right))) {
                System.out.print(cur.val + " ");
                pre = s.pop();
            }else {
                if(cur.right != null) s.push(cur.right);
                if(cur.left != null) s.push(cur.left);
            }
        }
        
    }
    
    public static void inOrder(TreeNode root) {
        Stack<TreeNode> s = new Stack<>();
        TreeNode node = root;
        
        while(node != null || !s.isEmpty()) {
            if(node != null) {
                s.push(node);
                node = node.left;
            }else {
                node = s.pop();
                System.out.print(node.val + " ");
                node = node.right;
            }
        }
    }
    
    public static void preOrder(TreeNode root) {
        Stack<TreeNode> s = new Stack<>();
        TreeNode node = root;
        
        while(node != null || !s.isEmpty()) {
            if(node != null) {
                System.out.print(node.val + " ");
                s.push(node);
                node = node.left;
            }else {
                node = s.pop();
                node = node.right;
            }
        }
    }
    
    
    
    public static void preOrderR(TreeNode root) {
        if(root == null) return ;
        System.out.print(root.val + " ");
        preOrderR(root.left);
        preOrderR(root.right);
    }
    
    public static void inOrderR(TreeNode root) {
        if(root == null) return ;
        
        inOrderR(root.left);
        System.out.print(root.val + " ");
        inOrderR(root.right);
    }
    
    public static void posOrderR(TreeNode root) {
        if(root == null) return ;
        
        posOrderR(root.left);
        
        posOrderR(root.right);
        System.out.print(root.val + " ");
    }

}