95. Unique Binary Search Trees II

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

For example,
Given n = 3, your program should return all 5 unique BST’s shown below.

1 3 3 2 1
\ / / / \ \
3 2 1 1 3 2
/ / \ \
2 1 2 3
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题目是要将所有的二叉树输出来，思路是从1遍历到n，再调用递归，等于左子树乘以右子树的情况，SetTree定义了从start到end的生成的二叉树，当start>end,子树为null，当start=end，生成一个节点，当start

public ArrayList<TreeNode> generateTrees(int n) {
         ArrayList<TreeNode>list=new ArrayList<>();
            return SetTree(1, n);
      }
     private static ArrayList<TreeNode> SetTree(int start,int end) {
         ArrayList<TreeNode>list=new ArrayList<>();
         if(start>end){
             list.add(null);
             return list;
         }
         if (start==end) {
            list.add(new TreeNode(end));
            return list;
        }
         for (int i = start; i <= end ; i++) {
            ArrayList<TreeNode>left=SetTree(start, i-1);
            ArrayList<TreeNode>right=SetTree(i+1, end);
            for (int j = 0; j < left.size(); j++) {
                for (int k = 0; k < right.size(); k++) {
                    TreeNode node=new TreeNode(i);
                    node.left=left.get(j);
                    node.right=right.get(k);
                    list.add(node);
                }
            }
        }
        return list;
    }