Randomly pick a number from binary searach tree

public class Solution {
    public static int pickRandom(TreeNode root) {
        if (root == null) return -1;

        List<Integer> nums = getList(root);
        return nums.get(rand.nextInt(nums.size()));
    }

    public static List<Integer> getList(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) return result;

        List<Integer> left = getList(root.left);
        List<Integer> right = getList(root.right);
        result.addAll(left);
        result.add(root.val);
        result.addAll(right);
        return result;
    }
}

class Solution(object):
    def pickRandom(self, root):
        if not root:
            return

        stack, ret = [], []
        while stack or root:
            if root:
                stack.append(root)
                root = root.left
            else:
                root = stack.pop()
                ret.append(root)
                root = root.right
        return ret[random.randInt(0, len(ret) - 1)]