Unique Binary Search Tree 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 left, int right) {
List<TreeNode> result = new ArrayList<TreeNode>();
// 剪枝
if (left > right) {
result.add(null);
return result;
}

for (int i = left; i <= right; i++) {
List<TreeNode> leftTree = myGenerate(left, i - 1);
List<TreeNode> rightTree = myGenerate(i + 1, right);

for (TreeNode leftNode : leftTree) {
for (TreeNode rightNode : rightTree) {
TreeNode treeNode = new TreeNode(i);
treeNode.left = leftNode;
treeNode.right = rightNode;
result.add(treeNode);
}
}
}
return result;
}
}