递归与非递归的前序 中序 后序

import java.util.Stack;
/**
 * @author lEI
 * @date 2024/1/19
 */
 
public class PreInPosTraversal {
    public static class Node {
        public int value;
        public Node left;
        public Node right;

        public Node(int data) {
            this.value = data;
        }
    }
    // 递归先序遍历
    public static void preOrderRecur(Node head) {
        if (head == null) {
            return;
        }
        System.out.println(head.value);
        preOrderRecur(head.left);
        preOrderRecur(head.right);
    }
    // 递归中序遍历
    public static void inOrderRecur(Node head) {
        if (head == null) {
            return;
        }
        inOrderRecur(head.left);
        System.out.println(head.value);
        inOrderRecur(head.right);
    }
     // 递归后序遍历
    public static void posOrderRecur(Node head) {
        if (head == null) {
            return;
        }
        posOrderRecur(head.left);
        posOrderRecur(head.right);
        System.out.println(head.value);
    }
     // 非递归前序遍历
    public static void preOrderUnRecur(Node head) {
        System.out.print("pre-order: ");
        if (head != null) {
            Stack<Node> stack = new Stack<>();
            stack.push(head);
            while (!stack.isEmpty()) {
                head = stack.pop();
                System.out.println(head.value);
                if (head.right != null) {
                    stack.push(head.right);
                }
                if (head.left != null) {
                    stack.push(head.left);
                }
            }
        }
        System.out.println();
    }
    // 非递归中序遍历
    public static void inOrderUnRecur(Node head) {
        System.out.print("in-order: ");
        if (head != null) {
            Stack<Node> stack = new Stack<>();
            while (!stack.isEmpty() || head != null) {
                if (head != null) {
                    stack.push(head);
                    head = head.left;
                }else {
                    head = stack.pop();
                    System.out.println(head.value);
                    head = head.right;
                }
            }
        }
        System.out.println();
    }
     // 非递归后序遍历
    public static void posOrderUnRecur(Node head) {
        System.out.print("pos-order: ");
        if (head != null) {
            Stack<Node> s1 = new Stack<>();
            Stack<Node> s2 = new Stack<>();
            s1.push(head);
            while (!s1.isEmpty()) {
                head = s1.pop();
                s2.push(head);
                if (head.left != null) {
                    s1.push(head.left);
                }
                if (head.right != null) {
                    s1.push(head.right);
                }
            }
            while (!s2.isEmpty()) {
                System.out.println(s2.pop().value + " ");
            }
        }
        System.out.println();
    }
     // 非递归后序遍历
    public static void posOrderUnRecur2(Node head) {
        System.out.print("pos-order: ");
        if (head != null) {
            Stack<Node> stack = new Stack<>();
            stack.push(head);
            Node cur = null;
            while (!stack.isEmpty()) {
                cur = stack.peek();
                if (cur != null && head != cur.left && head != cur.right) {
                    stack.push(cur.left);
                }else if (cur.right != null && cur.right != head) {
                    stack.push(cur.right);
                }else {
                    System.out.println(stack.pop() + " ");
                    head = cur;
                }
            }
        }
        System.out.println();
    }
    public static void main(String[] args) {
        Node head = new Node(5);
        head.left = new Node(3);
        head.right = new Node(8);
        head.left.left = new Node(2);
        head.left.right = new Node(4);
        head.left.left.left = new Node(1);
        head.right.left = new Node(7);
        head.right.left.left = new Node(6);
        head.right.right = new Node(10);
        head.right.right.left = new Node(9);
        head.right.right.right = new Node(11);

        // recursive
        System.out.println("==============recursive==============");
        System.out.print("pre-order: ");
        preOrderRecur(head);
        System.out.println();
        System.out.print("in-order: ");
        inOrderRecur(head);
        System.out.println();
        System.out.print("pos-order: ");
        posOrderRecur(head);
        System.out.println();

        // unrecursive

    }
}