Preorder traversal: Root, Left, Right
Solution 1, using single while loop.

public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> result = new ArrayList<>();
    Deque<TreeNode> stack = new ArrayDeque<>();
    TreeNode p = root;
    while(!stack.isEmpty() || p != null) {
        if(p != null) {
            stack.push(p);
            result.add(p.val);  // Add before going to children
            p = p.left;
        } else {
            TreeNode node = stack.pop();
            p = node.right;   
        }
    }
    return result;
}

Solution 2, nested while loop

public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<Integer>();
        if (root == null) return list;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode cur = root;
        while (cur != null || !stack.empty()) {
            while (cur != null) {
                list.add(cur.val);
                if (cur.right != null) {
                    stack.add(cur.right);
                }
                cur = cur.left;
            }
            if (!stack.empty()) {
                cur = stack.pop();
            }
        }
        return list;
    }

Inorder Traversal: Left, Root, Right

// solution 1
public List<Integer> inorderTraversal(TreeNode root) {
    List<Integer> result = new ArrayList<>();
    Deque<TreeNode> stack = new ArrayDeque<>();
    TreeNode p = root;
    while(!stack.isEmpty() || p != null) {
        if(p != null) {
            stack.push(p);
            p = p.left;
        } else {
            TreeNode node = stack.pop();
            result.add(node.val);  // Add after all left children
            p = node.right;   
        }
    }
    return result;
}

// solution 2
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        if (root == null) return list;
        TreeNode cur = root;
        while (cur != null || !stack.empty()) {
            while (cur != null) {
                stack.add(cur);  
                cur = cur.left;
            }
            cur = stack.pop();
            list.add(cur.val);
            cur = cur.right;
        }
        return list;
    }
}

Postorder Traversal: Left, Right, Root

public List<Integer> postorderTraversal(TreeNode root) {
    LinkedList<Integer> result = new LinkedList<>();
    Deque<TreeNode> stack = new ArrayDeque<>();
    TreeNode p = root;
    while(!stack.isEmpty() || p != null) {
        if(p != null) {
            stack.push(p);
            result.addFirst(p.val);  // Reverse the process of preorder
            p = p.right;             // Reverse the process of preorder
        } else {
            TreeNode node = stack.pop();
            p = node.left;           // Reverse the process of preorder
        }
    }
    return result;
}