前序

144. 二叉树的前序遍历

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        while(!stack.isEmpty() || root != null){
            while(root != null){
                res.add(root.val);
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            root = root.right;
        }
        return res;
    }
}

中序

94. 二叉树的中序遍历

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        while(!stack.isEmpty() || root != null){
            while(root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            res.add(root.val);
            root = root.right;
        }
        return res;
    }
}

后序

145. 二叉树的后序遍历

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        TreeNode pre = null;
        while(!stack.isEmpty() || root != null){
            while(root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            // 判断是否访问过右子树（没有右子树或有右子树且已经访问过）
            if(root.right == null || pre == root.right){
                res.add(root.val);
                pre = root;
                root = null;
            }else{  // 如果没有访问过右子树 则进行访问
                stack.push(root);
                root = root.right;
            }
        }
        return res;
    }
}