【leetcode】Unique Binary Search Trees II

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

 

 

 

从1-n中选取一个元素i，i左边的元素都在左子树上，i右边的元素都在右子树上

 

/**
 * 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 *> generateTrees(int n) {
       
        vector<TreeNode *>result=build(1,n);
        return result;
    }
   
    vector<TreeNode *> build(int l,int r)
    {
       
        if(l>r)
        {
            vector<TreeNode *> root(1);
            root[0]=NULL;
            return root;
        }
       
        vector<TreeNode *> result;
       
        for(int i=l;i<=r;i++)
        {
            vector<TreeNode *> left=build(l,i-1);
            vector<TreeNode *> right=build(i+1,r);
            for(int j=0;j<left.size();j++)
            {
                for(int k=0;k<right.size();k++)
                {
                    TreeNode *root=new TreeNode(i);
                    root->left=left[j];
                    root->right=right[k];
                    result.push_back(root);
                   
                }
            }
        }
       
        return result;
    }
};