95. Unique Binary Search Trees II 

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class Solution {
    public List<TreeNode> generateTrees(int n) {
        return generateTrees(1, n);
    }

    /**
     * @param start inclusive
     * @param end   inclusive
     */
    private List<TreeNode> generateTrees(int start, int end) {
        List<TreeNode> treeList = new ArrayList<>();
        if (start > end) {
            treeList.add(null);
            return treeList;
        }

        for (int root = start; root <= end; root++) {
            List<TreeNode> leftTrees = generateTrees(start, root - 1);
            List<TreeNode> rightTrees = generateTrees(root + 1, end);

            for (TreeNode leftTreeRoot : leftTrees) {
                for (TreeNode rightTreeRoot : rightTrees) {
                    TreeNode rootNode = new TreeNode(root);
                    rootNode.left = leftTreeRoot;
                    rootNode.right = rightTreeRoot;
                    treeList.add(rootNode);
                }
            }
        }

        return treeList;
    }
}

References

95. Unique Binary Search Trees II