题目：

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

 

confused what "{1,#,2,3}" means? 

题目：

这题要求将每个子树输出，思想就是对于n，以1-n为根节点，然后左右子树分别是比其小和比其大的节点构成的，对左右子树的的形态也要使用遍历，代码中使用的是foreach遍历：for(i:j)这种形式

1 /** 2  * Definition for a binary tree node. 3  * struct TreeNode { 4  *     int val; 5  *     TreeNode *left; 6  *     TreeNode *right; 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8  * }; 9  */10 class Solution {11 public:12     vector
     
       generateTrees(int n) {13         if (n == 0) return generate(1, 0);14         return generate(1, n);15     }16 private:17     vector
      
        generate(int start, int end) {18         vector
       
         subTree;19         if (start > end) {20             subTree.push_back(nullptr);21             return subTree;22         }23         for (int k = start; k <= end; k++) {24             vector
        
          leftSubs = generate(start, k - 1);25             vector
         
           rightSubs = generate(k + 1, end);26             for (auto i : leftSubs) {27                 for (auto j : rightSubs) {28                     TreeNode *node = new TreeNode(k);29                     node->left = i;30                     node->right = j;31                     subTree.push_back(node);32                 }33             }34         }35         return subTree;36     }37 };