Unique Binary Search Trees II

/**
* 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) {
return myGenerate(1, n);
}
private List<TreeNode> myGenerate(int start, int end) {
List<TreeNode> result = new ArrayList<TreeNode>();
//截止条件
if (start > end) {
result.add(null);
return result;
}
for (int i = start; i <= end; i++) {
List<TreeNode> left = myGenerate(start, i - 1);
List<TreeNode> right = myGenerate(i + 1, end);
for (TreeNode l : left) {
for (TreeNode r : right) {
TreeNode root = new TreeNode(i);
root.left = l;
root.right = r;
result.add(root);
}
}
}
return result;
}
}