java 95.独特的二进制搜索树II（＃）。java

Posted 2021-05-08

/**
 * 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) {
        if (n == 0) return new ArrayList<TreeNode>();
        return genTrees(1,n);
    }
        
    public List<TreeNode> genTrees (int start, int end)
    {

        List<TreeNode> list = new ArrayList<TreeNode>();

        if(start>end)
        {
            list.add(null);
            return list;
        }
        
        if(start == end){
            list.add(new TreeNode(start));
            return list;
        }
        
        List<TreeNode> left,right;
        for(int i=start;i<=end;i++)
        {
            
            left = genTrees(start, i-1);
            right = genTrees(i+1,end);
            
            for(TreeNode lnode: left)
            {
                for(TreeNode rnode: right)
                {
                    TreeNode root = new TreeNode(i);
                    root.left = lnode;
                    root.right = rnode;
                    list.add(root);
                }
            }
                
        }
        
        return list;
    }
}