[LeetCode]Unique Binary Search Trees II

最新推荐文章于 2020-12-09 16:51:01 发布

原创最新推荐文章于 2020-12-09 16:51:01 发布 · 1.1k 阅读

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本文介绍了一种使用递归方法生成所有可能的不同形态的二叉搜索树的方法。通过定义TreeNode结构体来表示二叉树节点，并利用C++类实现了一个Solution类，该类中包含两个关键函数：generateTrees和generateUtil。这些函数能够生成给定数量的节点所能组成的全部不重复的二叉搜索树。

struct TreeNode {
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution {
public:
	vector<TreeNode*> GenerateTrees(int lower, int higher)
	{
		vector<TreeNode*> res;
		if(lower > higher)
		{
			res.push_back(NULL);
			return res;
		}
		for (int i = lower; i <= higher; ++i)
		{
			vector<TreeNode*> leftTree = GenerateTrees(lower, i-1);
			vector<TreeNode*> rightTree = GenerateTrees(i+1, higher);
			for (int l = 0; l < leftTree.size(); ++l)
			{
				for (int r = 0; r < rightTree.size(); ++r)
				{
					TreeNode* root = new TreeNode(i);
					root->left = leftTree[l];
					root->right = rightTree[r];
					res.push_back(root);
				}
			}
		}
		return res;
	}
	vector<TreeNode *> generateTrees(int n) {
		// Start typing your C/C++ solution below
		// DO NOT write int main() function
		return GenerateTrees(1, n);
	}
};

second time

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<TreeNode *> generateUtil(int start, int end)
    {
        vector<TreeNode *> ans;
        if(start > end)
        {
            ans.push_back(NULL);
            return ans;
           
        }
        
        for(int i = start; i <= end; ++i)
        {
            vector<TreeNode*> leftTrees = generateUtil(start, i-1);
            vector<TreeNode*> rightTrees = generateUtil(i+1, end);
            for(int l = 0; l < leftTrees.size(); ++l)
            {
                for(int r = 0; r < rightTrees.size(); ++r)
                {
                    TreeNode* root = new TreeNode(i);
                    root->left = leftTrees[l];
                    root->right = rightTrees[r];
                    ans.push_back(root);
                }
            }
        }
        return ans;
    }
    vector<TreeNode *> generateTrees(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        return generateUtil(1, n);
    }
};