import java.util.Stack;
public class prepos {
  public static void main(String[] args) {}

  public static class Node {
    public int value;
    public Node left;
    public Node right;

    Node(int data) { this.value = data; }
  }

  //方法一:递归
  //先序遍历
  public static void preOrderRecur(Node head) {
    if (head == null) {
      return;
    }
    System.out.print(head.value);
    preOrderRecur(head.left);
    preOrderRecur(head.right);
  }

  //中序遍历
  public static void inOrderRecur(Node head) {
    if (head == null) {
      return;
    }
    inOrderRecur(head.left);
    System.out.print(head.value);
    inOrderRecur(head.right);
  }

  //后序遍历
  public static void postOrderRecur(Node head) {
    if (head == null) {
      return;
    }
    postOrderRecur(head.left);
    postOrderRecur(head.right);
    System.out.println(head.value);
  }
  //方法二:使用栈
  //先序遍历
  public static void pretOrderunRecur(Node head) {
    if (head != null) {
      Stack<Node> stack = new Stack<Node>();
      stack.add(head);
      while (!stack.isEmpty()) {
        head = stack.pop();
        System.out.print(head.value);
        if (head.right != null) {
          stack.push(head.right);
        }
        if (head.left != null) {
          stack.push(head.left);
        }
      }
    }
  }
  //中序遍历
  public static void inOrderunRecur(Node head) {
    if (head != null) {
      Stack<Node> stack = new Stack<Node>();
      while (!stack.isEmpty() || head != null) {
        if (head != null) {
          stack.push(head);
          head = head.left;
        } else {
          head = stack.pop();
          System.out.print(head.value);
          head = head.right;
        }
      }
    }
  }
  //后序遍历
  public static void postOrderUnRecur(Node head) {
    if (head != null) {
      Stack<Node> stack1 = new Stack<Node>();
      Stack<Node> stack2 = new Stack<Node>();
      stack1.push(head);
      while (!stack1.isEmpty()) {
        head = s1.pop();
        stack2.push(head);
        if (head.left != null) {
          stack1.push(head.left);
        }
        if (head.right != null) {
          stack1.push(head.right);
        }
      }
      while (!stack2.isEmpty()) {
        System.out.print(stack2.pop().value);
      }
    }
  }
  //后序遍历只用一个栈
  public static void postOrderRecur2(Node h) {
    if (h != null) {
      Stack<Node> stack = new Stack<Node>();
      stack.push(h);
      Node c = null;
      while (!stack.isEmpty()) {
        c = stack.peek();
        if (c.left != null && h != c.left && h != c.right) {
          stack.push(c.left);
        } else if (c.right != null && h != c.right) {
          stack.push(c.right);
        } else {
          System.out.print(stack.pop().value);
          h = c;
        }
      }
    }
  }
}