递归方式实现二叉树的先序、中序、后序遍历：
理解递归序：递归遍历的本质是递归序，二叉树递归序，每个节点都会达到三次。先序、中序、后序都可以在递归序的基础上加工出来，第一次到达一个节点就打印就是先序、第二次打印即中序、第三次即后序。
package com.harrison.class07; public class Code01_RecursiveTraversalBT { public static class Node{ public int value; public Node left; public Node right; public Node(int v) { value=v; } } public static void f(Node head) { if(head==null) { return ; } f(head.left); f(head.right); } public static void pre(Node head) { if(head==null) { return ; } System.out.print(head.value+" "); pre(head.left); pre(head.right); } public static void in(Node head) { if(head==null) { return ; } in(head.left); System.out.print(head.value+" "); in(head.right); } public static void pos(Node head) { if(head==null) { return ; } pos(head.left); pos(head.right); System.out.print(head.value+" "); } 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); pre(head); System.out.println("\n========"); in(head); System.out.println("\n========"); pos(head); System.out.println("\n========"); } }
