[数据结构与算法]非递归方式实现二叉树的先序遍历、后续遍历

先序遍历：弹出就打印；如有右孩子压入右孩子；如有左孩子，压入左孩子

public static void pre(Node head) {
		System.out.print("pre-ord");
		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);
				}
			}
		}
		System.out.println();
	}

后续遍历：准备两个栈，每一个头结点弹出后用另一个栈s2收集，先压左，再压右，最后s2弹出（即先序遍历改为先压左后压右，最后逆序输出）

public static void pos(Node head) {
		System.out.print("pos-order");
		if(head!=null) {
			Stack<Node> s1=new Stack<Node>();
			Stack<Node> s2=new Stack<Node>();
			s1.push(head);
			while(!s1.isEmpty()) {
				head=s1.pop();
				s2.push(head);
				if(head.left!=null) {
					s1.push(head.left);
				}
				if(head.right!=null) {
					s2.push(head.right);
				}
			}
			while(!s2.isEmpty()) {
				System.out.print(s2.pop().value+"");
			}
		}
		System.out.printil();
	}