LeetCode 95. Unique Binary Search Trees II

95. Unique Binary Search Trees II

相关链接

leetcode

描述

  Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1 ... n.

Example:

Input: 3
Output:
[
[1,null,3,2],
[3,2,null,1],
[3,1,null,null,2],
[2,1,3],
[1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST's shown below:

   1         3     3      2      1
           /     /      /       
     3     2     1      1   3      2
    /     /                        
   2     1         2                 3

思路1递归解决

按照根节点来分类的话，可以分成n类；
假设要生成根节点为i的所有BST，可以先生成（0 到 i-1）的所有左子树和（i+1到n）的所有右子树，然后两两组合；

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<TreeNode*> generateTrees(int n) {
        vector<TreeNode*> tree;
        if (n == 0)
            return tree;
        dfs(1, n, tree);
        return tree;
    }

    void dfs(int start, int end, vector<TreeNode*> &tree)
    {
        if (start > end)
        {
            tree.push_back(NULL);
        }

     for(int i = start; i <= end; i++){
         vector<TreeNode*> left;
         vector<TreeNode*> right;

         dfs(start, i-1, left);
         dfs(i+1, end, right);

         for(int k = 0; k < left.size(); k++)
             for(int j = 0; j < right.size(); j++)
             {
                 TreeNode * root = new TreeNode(i);
                 root->left = left[k];
                 root->right = right[j];

                 tree.push_back(root); 
             }
    }
};

参考
csdn