preorder . 144

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        if (root == null) {
            return ans;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode head = stack.pop();
            if (head.right != null) {
                stack.push(head.right);
            }
            if (head.left != null) {
                stack.push(head.left);
            }
            ans.add(head.val);
        }
        return ans;
    }
}

inorder

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

    }
}

postorder

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> ans = new LinkedList<>();
        if (root == null) {
            return ans;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode curt = stack.pop();
            ans.add(0, curt.val);
            if (curt.left != null) {
                stack.push(curt.left);
            }
            if (curt.right != null) {
                stack.push(curt.right);
            }
        }
        return ans;

    }
}

class Solution {
    int ans;
    int count;
    public int kthSmallest(TreeNode root, int k) {
        count = k;
        helper(root);
        return ans;
    }
    public void helper(TreeNode root) {
        if (root == null || count == 0) {
            return;
        }
        helper(root.left);
        count--;
        if (count == 0) {
            ans = root.val;
        }
        helper(root.right);
    }
}