前序遍历

 public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>(16);

        if (null == root) {
             return res;
        }

        List<Integer> lRes = preorderTraversal(root.left);
        List<Integer> rRes = preorderTraversal(root.right);
        res.add(root.val);
        res.addAll(lRes);
        res.addAll(rRes);

        return res;
    }

另一种写法： 使用栈进行迭代

public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>(16);

        if (null == root) {
            return res;
        }

        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.empty()) {
            TreeNode node = stack.pop();
            if (null != node.right) {
                stack.push(node.right);
            }

            if (null != node.left) {
                stack.push(node.left);
            }

            res.add(node.val);
        }

        return res;
    }

中序遍历

常见写法 递归

    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        List<Integer> left = inorderTraversal(root.left);
        List<Integer> right = inorderTraversal(root.right);

        result.addAll(left);
        result.add(root.val);
        result.addAll(right);
        return result;
    }

另一种写法： 使用栈进行迭代

public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        TreeNode current = root;
        while (current != null || !stack.isEmpty()) {
            while(current != null) {
                stack.add(current);
                current = current.left;
            }
            current = stack.pop();
            result.add(current.val);
            current = current.right;
        }
        return result;
    }

后序遍历
常见递归写法

    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        List<Integer> left = postorderTraversal(root.left);
        List<Integer> right = postorderTraversal(root.right);

        result.addAll(left);
        result.addAll(right);
        result.add(root.val);
        return result;
    }

使用栈进行迭代

    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.peek();
            if (node.right == null && node.left == null) {
                result.add(stack.pop().val);
            }
            if (node.right != null) {
                stack.push(node.right);
                node.right = null;
            }
            if (node.left != null) {
                stack.push(node.left);
                node.left = null;
            }
        }
        return result;
    }