不同的二叉搜索树 II

不同的二叉搜索树 II

题目：

示例 1：

输入：n = 3
输出：[[1,null,2,null,3],[1,null,3,2],[2,1,3],[3,1,null,null,2],[3,2,null,1]]

示例 2：
输入：n = 1
输出：[[1]]

解题思路：用dfs进行解决，分别遍历以[1, n]为根节点时形成的二叉树，得到它的左子节点和右子节点的组成后，进行组合

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<TreeNode> generateTrees(int n) {
        if(n == 0) {
            return new ArrayList<TreeNode>();
        }

        return dfs(1, n);
    }

    private List<TreeNode> dfs(int start, int end) {
        List<TreeNode> ans = new ArrayList();
        if(start > end) {
            ans.add(null);
            return ans;
        }

        if(start == end) {
            ans.add(new TreeNode(start));
            return ans;
        }

        for(int i = start; i <= end; i++) {
            List<TreeNode> left = dfs(start, i - 1);
            List<TreeNode> right = dfs(i + 1, end);
            for(TreeNode l : left) {
                for(TreeNode r : right) {
                    TreeNode cur = new TreeNode(i);
                    cur.left = l;
                    cur.right = r;
                    ans.add(cur);
                }
            }
        }

        return ans;
    }
}