后序遍历：双栈法，和层次遍历（双队列）很相似，唯一区别在于层次遍历用的 是队列，后序遍历用的是栈。

public static void posOrderUnRecur1(Node head){
	System.out.print("PosOrder:");
	if(head != null){
		Stack<Node> s1 = new Stack<Node>();
		Stack<Node> s2 = new Stack<Node>();
		si.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.print(s2.pop().value + " ");
		}
	}
	System.out,println();
}
 
public static void posOrderUnRecur2(Node h){
	Sytem.out.print("PosOrder:");
	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;
			}
		}
	}
	System.out.println();
}

参考：https://blog.csdn.net/davidddl/article/details/75667092