前中后序遍历树--递归非递归方法Java实现

import java.util.Stack;

class TreeNode {
     int val;
     TreeNode left;
     TreeNode right;
     TreeNode(int x) { val = x; }
}
public class Tree {

    //递归先序
    public void REPreOrder(TreeNode root) {
        if (root == null) return;
        System.out.println(root.val);
        REPreOrder(root.left);
        REPreOrder(root.right);
    }

    //非递归先序 栈中保存树的右节点
    public void PreOrderA(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode p = root;
        while (p != null) {
            System.out.println(p.val);
            if (p.right != null) stack.push(p);
            if (p.left != null) p = p.left;
            else p = stack.pop();
        }
    }

    //非递归先序 栈中保存树的左子树
    public void PreOrderB(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                System.out.println(root.val);
                root = root.left;
            }
            if (!stack.isEmpty()) {
                root = stack.pop();
                root = root.right;
            }
        }
    }

    //递归中序
    public void REInOrder(TreeNode root) {
        if (root == null) return;
        REPreOrder(root.left);
        System.out.println(root.val);
        REPreOrder(root.right);
    }
    
    //非递归中序 栈中保存树的左子树
    public void InOrder(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            if (!stack.isEmpty()) {
                root = stack.pop();
                System.out.println(root.val);
                root = root.right;
            }
        }
    }

    //递归后序
    public void REPostOrder(TreeNode root) {
        if (root == null) return;
        REPreOrder(root.left);
        REPreOrder(root.right);
        System.out.println(root.val);
    }
    
    //非递归后序 增加标志位栈
    /**
     * 1. 当root非空时循环：将root和标志1压入栈中，继续遍历左子树
     * 2. 当栈非空且栈顶元素的标志位为2，输出该元素
     * 3. 当栈非空将栈顶元素的标志位改为2（表示已经为第二次遍历），将root设置为栈顶元素的右子树
     * @param root
     */
    public void PostOrderA(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        //标志栈
        Stack<Integer> flag = new Stack<>();
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                flag.push(1);
                root = root.left;
            }
            while (!stack.isEmpty() && flag.peek() == 2) {
                System.out.println(stack.pop().val);
                flag.pop();
            }
            //第二次遍历到
            if (!stack.isEmpty()) {
                flag.pop();
                flag.push(2);
                root = stack.peek().right;
            }
        }
    }

}