Leetcode95. Unique Binary Search Trees II不同的二叉搜索树2

给定一个整数 n，生成所有由 1 ... n 为节点所组成的二叉搜索树。

示例:

输入: 3

输出: [   [1,null,3,2],   [3,2,null,1],   [3,1,null,null,2],   [2,1,3],   [1,null,2,null,3] ]

重建树一般都是递归求解

class Solution {
public:
    vector<TreeNode*> generateTrees(int n) 
    {
        if(n == 0)
            return {};
        return GetAns(1, n);
    }

    vector<TreeNode*> GetAns(int start, int end)
    {
        vector<TreeNode*> subTree;
        if(start > end)
        {
            subTree.push_back(NULL);
        }
        else
        {
            for(int i = start; i <= end; i++)
            {
                vector<TreeNode*> left = GetAns(start, i - 1);
                vector<TreeNode*> right = GetAns(i + 1, end);
                for(auto l : left)
                {
                    for(auto r : right)
                    {
                        TreeNode *root = new TreeNode(i);
                        root ->left = l;
                        root ->right = r;
                        subTree.push_back(root);
                    }
                }
            }
        }
        return subTree;
    }
};