【leetcode刷题笔记】Unique Binary Search Trees II

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

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题解：递归的枚举1~n的每个节点为根节点，然后递归的利用它左边的节点构造左子树，放在一个list里面；再利用它右边的节点构造右子树，也放在一个list里面；最终枚举两个list里面的左子树和右子树，构建一棵树。

代码如下：

/**
 * 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 {
    private ArrayList<TreeNode> generate(int start,int end){
        ArrayList<TreeNode> answer = new ArrayList<TreeNode>();
        if(start > end){
            answer.add(null);
            return answer;
        }
        
        //for every node from start to right,make it as tree root and recursively build its left and right child tree
        for(int i = start; i <= end;i++){
            ArrayList<TreeNode> left = generate(start, i-1);
            ArrayList<TreeNode> right = generate(i+1, end);
            for(TreeNode l:left){
                for(TreeNode r:right){
                    TreeNode root = new TreeNode(i);
                    root.left = l;
                    root.right = r;
                    answer.add(root);
                }
            }
            
        }
        
        return answer;
        
    }
    public List<TreeNode> generateTrees(int n) {
        return generate(1,n);
    }
}