二叉树的相关代码

1、二叉树的先序，中序，后序遍历

import java.util.Stack;
/**
 * @Author zb
 * @Date 2018/8/14
 * 二叉树的先序，中序，后序遍历
 */
public class PreInPosTraversal {

    /**
     * 定义二叉树结点
     */
    public static class Node{
        public int value;
        public Node left;
        public Node right;
        public Node(int data){
            this.value = data;
        }
    }

    /**
     * 递归实现先序遍历
     * @param head
     */
    public static void preOrderRecur(Node head){
        if(head == null){
            return ;
        }
        System.out.print(head.value + " ");
        preOrderRecur(head.left);
        preOrderRecur(head.right);
    }

    /**
     * 递归实现中序遍历
     * @param head
     */
    public static void inOrderRecur(Node head){
        if(head == null){
            return ;
        }
        inOrderRecur(head.left);
        System.out.print(head.value + " ");
        inOrderRecur(head.right);
    }

    /**
     * 递归实现后序遍历
     * @param head
     */
    public static void posOrderRecur(Node head){
        if(head == null){
            return ;
        }
        posOrderRecur(head.left);
        posOrderRecur(head.right);
        System.out.print(head.value + " ");
    }

    /**
     * 非递归实现先序遍历
     * @param head
     */
    public static void preOrderUnRecur(Node head){
        System.out.println("pre-order: ");
        if(head != null){
            Stack<Node> stack = new Stack<Node>();
            //向栈中添加头结点
            stack.add(head);
            //当前结点
            Node cur;
            while(!stack.isEmpty()){
                cur = stack.pop();
                System.out.print(cur.value + " ");
                //注意：如果不为空，先压右，再压左(弹栈打印的时候才会是先左后右)
                if(cur.right != null){
                    stack.push(cur.right);
                }
                if(cur.left != null){
                    stack.push(cur.left);
                }
            }
        }
        System.out.println();
    }

    /**
     * 非递归实现中序遍历
     * @param head
     */
    public static void inOrderUnRecur(Node head){
        System.out.println("in-order：");
        if(head != null){
            Stack<Node> stack = new Stack<Node>();
            Node cur = head;
            //当前结点为空：从栈中拿一个打印，当前结点向右跑
            //当前结点不为空，当前结点压入栈，当前结点向左跑
            while(!stack.isEmpty() || cur != null){
                if(cur != null){
                    stack.push(cur);
                    cur = cur.left;
                }else{
                    cur = stack.pop();
                    System.out.print(cur.value + " ");
                    cur = cur.right;
                }
            }
        }
        System.out.println();
    }

    /**
     * 非递归实现后序遍历(两个栈)
     * @param head
     */
    public static void posOrderUnRecur1(Node head){
        System.out.println("pos-order：");
        if(head != null){
            Stack<Node> s1 = new Stack<Node>();
            Stack<Node> s2 = new Stack<Node>();
            s1.push(head);
            Node cur = head;
            //根据先序遍历(中左右)的代码,求出(中右左)，再根据栈的特性，逆过来就是后序(左右中)
            //s2作为辅助栈,把先序遍历中要打印的结点，先压入栈中,最后打印，即为左右中
            while (!s1.isEmpty()){
                cur = s1.pop();
                s2.push(cur);
                if(cur.left != null){
                    s1.push(cur.left);
                }
                if(cur.right != null){
                    s1.push(cur.right);
                }
            }
            //打印
            while (!s2.isEmpty()){
                System.out.print(s2.pop().value + " ");
            }
        }
        System.out.println();
    }

    /**
     * 非递归实现后序遍历(一个栈)
     * @param head
     */
    public static void posOrderUnRecur2(Node head){
        System.out.println("pos-order：");
        if(head != null){
            Stack<Node> stack = new Stack<Node>();
            stack.push(head);
            Node cur = null;
            while(!stack.isEmpty()){
                cur = stack.peek();
                if(cur.left != null && head != cur.left && head != cur.right){
                    stack.push(cur.left);
                }else if(cur.right != null && head != cur.right){
                    stack.push(cur.right);
                }else {
                    System.out.print(stack.pop().value + " ");
                    head = cur;
                }
            }
        }
        System.out.println();
    }

    public static void main(String[] args) {
        Node head = new Node(1);
        head.left = new Node(2);
        head.right = new Node(3);
        head.left.left = new Node(4);
        head.left.right = new Node(5);
        head.right.left = new Node(6);
        head.right.right = new Node(7);

        // 递归实现
        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();

        //非递归实现
        System.out.println("============unrecursive=============");
        preOrderUnRecur(head);
        inOrderUnRecur(head);
        posOrderUnRecur1(head);
        posOrderUnRecur2(head);
    }

}