解题思路：

全部插入到叶子节点即可

class Solution {
    public TreeNode insertIntoBST(TreeNode root, int val) {
        if (root == null) {
            TreeNode node = new TreeNode(val);
            return node;
        }

        if (root.val < val) {
            root.right = insertIntoBST(root.right, val);
        }
        if (root.val > val) {
            root.left = insertIntoBST(root.left, val);
        }
        return root;
    }
}