LeetCode_DP_Unique Binary Search Trees II

Unique Binary Search Trees II

---

一、题目说明

与 Unique Binary Search Trees（http://blog.youkuaiyun.com/gldemo/article/details/48167973）不同的地方是输出所有可能组成的二叉搜索树。

二、思路

该算法应该是采用循环，循环按每个 i 作为根节点，循环中应用递归的方式去不断构造左子树和右子树。然后在循环中组合不同的左右子树。

三、java代码

/**
 * 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> generateTress(int n) {
        return generateTrees(1, n);
    }

    public List<TreeNode> generateTrees(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 = generateTrees(start, i-1);
            right = generateTrees(i+1, end);
            for(TreeNode l : left) {
                for(TreeNode r : right) {
                    TreeNode root = new TreeNode(i);
                    root.left = l;
                    root.right = r;
                    list.add(root);
                }
            }
        }
        return list;
    }
}