95. Unique Binary Search Trees II

{leetcode}/problems/unique-binary-search-trees-ii/[LeetCode - Unique Binary Search Trees II^]

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

Example:

Input: 3
Output:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST's shown below:

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

link:{sourcedir}/_0095_UniqueBinarySearchTreesII.java[role=include]

这道题用到了回溯思想！昨天突击，又重新看了一遍回溯。有个模糊影响，但是还是要加强练习。