学步园

Given 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

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; left = null; right = null; }
 * }
 */
public class Solution {
    public ArrayList<TreeNode> generateTrees(int n) {
        // Start typing your Java solution below
        // DO NOT write main() function
        return get(1,n);
    }
    
    
    public ArrayList<TreeNode> get(int s,int e){
        ArrayList<TreeNode> res= new ArrayList<TreeNode>();
        if(s>e) {res.add(null);return res;}
        for(int i=s;i<=e;i++){
            ArrayList<TreeNode> l=get(s,i-1);
            ArrayList<TreeNode> r=get(i+1,e);

            for(int m=0;m<l.size();m++){
                for(int n=0;n<r.size();n++){
                    TreeNode c= new TreeNode(i);
                    c.left=l.get(m);
                    c.right=r.get(n);
                    res.add(c);
                }}
        }
        return res;
    }
}