以前只会二叉树的一种非递归遍历，最近又看了二叉树的多种非递归遍历方式，感觉很有用，亲自实现用理解了一番，总结如下。

一、常用方式

1、先序遍历

1.1 自己常用的一种方式：

public List<Integer> preOrder1(TreeNode root) {
		List<Integer> list = new ArrayList<>();
		Stack<TreeNode> stack = new Stack<>();
		stack.push(root);
		TreeNode node = null;
		while (!stack.isEmpty()) {
			node = stack.pop();
			if (node != null) {
				list.add(node.value);
				stack.push(node.right);
				stack.push(node.left);
			}
		}
		return list;
	}

public List<Integer> preOrder2(TreeNode root) {
		List<Integer> list = new ArrayList<>();
		Stack<TreeNode> stack = new Stack<>();
		TreeNode node = root;
		while (!stack.isEmpty() || node !=null) {
			if(node != null){
				list.add(node.value);
				stack.push(node);
				node = node.left;				
			} else {
				node = stack.pop();
				node = node.right;
			}
		}
		return list;
	}

public List<Integer> inOrder1(TreeNode root) {
		List<Integer> list = new ArrayList<>();
		Stack<TreeNode> stack = new Stack<>();
		TreeNode node = root;
		while (!stack.isEmpty() || node !=null) {
			if(node != null){
				stack.push(node);
				node = node.left;				
			} else {
				node = stack.pop();
				list.add(node.value);
				node = node.right;
			}
		}
		return list;
	}

public List<Integer> postOrder(Node root) {
    	List<Integer> res = new ArrayList<>();
    	Stack<Node> stack = new Stack<>();
    	stack.push(root);
    	Node node = null;
    	Node pre = null; //访问的前一个元素
    	while(!stack.isEmpty()) {
    		node = stack.peek();
    		if(node != null) {
    			if(node.left != null && node.left != pre && node.right != pre) {
    				stack.push(node.left);
    			} else if(node.right != null && node.right != pre) {
    				stack.push(node.right);
    			} else {
    				res.add(node.value);
    				pre = stack.pop();  //此时 node = stack.pop();
    			} 			
    		}
    	}
    	return res;   
    }