95. Unique Binary Search Trees II

Problem statement:

Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1...n.

For example,
Given n = 3, your program should return all 5 unique BST's shown below.

   1         3     3      2      1
           /     /      /       
     3     2     1      1   3      2
    /     /                        
   2     1         2                 3

Solutions: 

Contrast with 96. Unique Binary Search Trees, this problem wants all possible solutions. Obviously, it is DFS.