Solution.java

// 递归版本
/*
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        inorderTraversal(root, list);
        return list;
    }
    
    private void inorderTraversal(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }
        inorderTraversal(root.left, list);
        list.add(root.val);
        inorderTraversal(root.right, list);
    }
    
}

*/

// 非递归版本
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            if (!stack.isEmpty()) {
                root = stack.pop();
                list.add(root.val);
                root = root.right;
            }
        }
        return list;
        
    }
    
}