二叉树的遍历

//定义一个树节点
class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}
 //非递归前序遍历
    public void preorderTraversal_1(TreeNode node,List<Integer> list){
        LinkedList<TreeNode> stack = new LinkedList<>();
        while(!stack.isEmpty() || node != null){
            while(node != null){
                //访问当前结点
                list.add(node.val);
                stack.push(node);
                node = node.left;
            }

            node = stack.pop();
            node = node.right;
        }
    }

//非递归的中序遍历
   //迭代算法 非递归写法 ， 用栈去模拟递归
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        Deque<TreeNode> stk = new LinkedList<TreeNode>();
        while (root != null || !stk.isEmpty()) {
            while (root != null) {
                stk.push(root);
                root = root.left;
            }
            root = stk.pop();
            res.add(root.val);
            root = root.right;
        }
        return res;
    }

//非递归的后序遍历 等价于 根右左的 反向序列
    public void postorderTraversal_1(TreeNode node,List<Integer> list) {
        Deque<TreeNode> stack = new LinkedList<>();
        Deque<TreeNode> reverse_res = new LinkedList<>();

        while(!stack.isEmpty() || node != null){
            while(node != null){
                stack.push(node);
                reverse_res.push(node);
                node = node.right;
            }
            node = stack.pop();
            node = node.left;
        }
        TreeNode temp_node = null;
        while(!reverse_res.isEmpty()){
            temp_node = reverse_res.pop();
            list.add(temp_node.val);
        }
    }