【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(,n);
         return result;
     }

     vector<TreeNode *> build(int l,int r)
     {

         if(l>r)
         {
             vector<TreeNode *> root();
             root[]=NULL;
             return root;
         }

         vector<TreeNode *> result;

         for(int i=l;i<=r;i++)
         {
             vector<TreeNode *> left=build(l,i-);
             vector<TreeNode *> right=build(i+,r);
             for(int j=;j<left.size();j++)
             {
                 for(int k=;k<right.size();k++)
                 {
                     TreeNode *root=new TreeNode(i);
                     root->left=left[j];
                     root->right=right[k];
                     result.push_back(root);

                 }
             }
         }

         return result;
     }
 };