（复习）[LeetCode]Unique Binary Search Trees II

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<TreeNode> generateTrees(int n) {
        if(n==0)
            return new LinkedList<>();
        return generate(1,n);
    }
    private List<TreeNode> generate(int start,int end){
        List<TreeNode> trees=new LinkedList<>();
        //base case
        if(start>end){
            trees.add(null);
            return trees;
        }

        for(int i=start;i<=end;i++){
            //get the Lists of  left and right subTree respectively
            List<TreeNode> leftSubs=generate(start,i-1);
            List<TreeNode> rightSubs=generate(i+1,end);
            for(TreeNode l:leftSubs)
                for(TreeNode r:rightSubs){
                    TreeNode root=new TreeNode(i);
                    root.left=l;
                    root.right=r;
                    trees.add(root);
                }
        }
        return trees;
    }
}