一篇文章教你如何用多种迭代写法实现二叉树遍历

    public List<Integer> preorderTraversal1(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (!stack.isEmpty() || root != null) {
            while (root != null) {
                res.add(root.val);
                stack.add(root);
                root = root.left;
            }
            TreeNode cur = stack.pop();
            root = cur.right;
        }
        return res;
    }
    public List<Integer> preorderTraversal2(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (!stack.isEmpty() || root != null) {
            if (root != null) {
                res.add(root.val);
                stack.add(root);
                root = root.left;
            } else {
                TreeNode cur = stack.pop();
                root = cur.right;
            }
        }
        return res;
    }
    public List<Integer> preorderTraversal3(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) return res;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            res.add(cur.val);
            if (cur.right != null) {
                stack.push(cur.right);
            }
            if (cur.left != null) {
                stack.push(cur.left);
            }
        }
        return res;
    }
    public List<Integer> preorderTraversal4(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            root = queue.poll();
            res.add(root.val);
            if (root.right != null) {
                queue.addFirst(root.right);
            }
            if (root.left != null) {
                root = root.left;
                while (root != null) {
                    res.add(root.val);
                    if (root.right != null) {
                        queue.addFirst(root.right);
                    }
                    root = root.left;
                }
            }
        }
        return res;
    }
    public List<Integer> inorderTraversal1(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (root != null || !stack.isEmpty()) {
            if (root != null) {
                stack.add(root);
                root = root.left;
            } else {
                TreeNode cur = stack.pop();
                res.add(cur.val);
                root = cur.right;
            }
        }
        return res;
    }
    public List<Integer> inorderTraversal2(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.add(root);
                root = root.left;
            }
            TreeNode cur = stack.pop();
            res.add(cur.val);
            root = cur.right;
        }
        return res;
    }
    public List<Integer> postorderTraversal1(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) return res;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            res.add(cur.val);
            if (cur.left != null) {
                stack.push(cur.left);
            }
            if (cur.right != null) {
                stack.push(cur.right);
            }
        }
        Collections.reverse(res);
        return res;
    }
    public List<Integer> postorderTraversal2(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (!stack.isEmpty()) {
            while (root != null) {
                res.add(root.val);
                stack.push(root);
                root = root.right;
            }
            TreeNode cur = stack.pop();
            root = cur.left;
        }
        Collections.reverse(res);
        return res;
    }
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ret = new ArrayList<>();
        if(root == null)return ret;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()){
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            while(size!=0){
                TreeNode cur = queue.poll();
                list.add(cur.val);
                if(cur.left!=null){
                    queue.offer(cur.left);
                }
                if(cur.right!= null){
                    queue.offer(cur.right);
                }
                size --;
            }
            ret.add(list);
        }
        return ret;
    }