Leetcode95. 不同的二叉搜索树 II

题目传送：​​https://leetcode.cn/problems/unique-binary-search-trees-ii/submissions/​​

运行效率：

代码如下：

public static List<TreeNode> generateTrees(int n) {
        List<TreeNode> treeNodes = generateTrees(1, n);
        return treeNodes;
    }

    /**
     * 将start到end之间的数字组合成搜索二叉树
     *
     * @param start
     * @param end
     * @return List里的每一个元素都是一颗搜索二叉树
     */
    public static List<TreeNode> generateTrees(int start, int end) {
        List<TreeNode> result = new ArrayList<>();
        //处理边界情况
        if (start == end) {
            TreeNode root = new TreeNode(start);
            result.add(root);
            return result;
        }

        //1、先确定二叉树根节点   (而根节点有可能是start到end之间的任何一个数字，所以我们枚举试试看)
        for (int i = start; i <= end; i++) {
            List<TreeNode> leftNodes = new ArrayList<>();
            List<TreeNode> rightNodes = new ArrayList<>();
            if (i - 1 >= start) {
                leftNodes = generateTrees(start, i - 1);
            }
            if (i + 1 <= end) {
                rightNodes = generateTrees(i + 1, end);
            }
            //二叉搜索树的左子树可以为空
            if (leftNodes.isEmpty()) {
                for (TreeNode right : rightNodes) {
                    TreeNode root = new TreeNode(i); //根节点
                    root.right = right;
                    result.add(root);
                }
            }
            //二叉搜索树的右子树可以为空
            if (rightNodes.isEmpty()) {
                for (TreeNode left : leftNodes) {
                    TreeNode root = new TreeNode(i); //根节点
                    root.left = left;
                    result.add(root);
                }
            }

            for (TreeNode left : leftNodes) {
                for (TreeNode right : rightNodes) {
                    TreeNode root = new TreeNode(i); //根节点
                    root.left = left;
                    root.right = right;
                    result.add(root);
                }
            }
        }
        return result;
    }