Java implementation LeetCode 95 different binary search tree II (b)

95. Different binary search tree II

Given an integer n, generated by the all 1 ... n is the binary search tree consisting of nodes.

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:
more than 5 different outputs corresponding to the binary search tree structure:

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

 
class Solution {
    public List<TreeNode> generateTrees(int n) {
    if(n == 0)
        return new LinkedList<TreeNode>();
    return generateTrees(1,n);
}
public List<TreeNode> generateTrees(int start,int end) {
    List<TreeNode> res = new LinkedList<TreeNode>();
    if(start > end){
        res.add(null);
        return res;
    }
    for(int i = start;i <= end;i++){
        List<TreeNode> subLeftTree = generateTrees(start,i-1);
        List<TreeNode> subRightTree = generateTrees(i+1,end);
        for(TreeNode left : subLeftTree){
            for(TreeNode right : subRightTree){
                TreeNode node = new TreeNode(i);
                node.left = left;
                node.right = right;
                res.add(node);
            }
        }                        
    }
    return res;
}
}