Unique Binary Search Trees II--Leetcode

Iven 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? > read more on how binary tree is serialized on OJ.

Basic idea: Recursion

1. Each element in turn is the root node

2. Recursively, all elements to the left of the element, returning a collection of sets of numbers.

3. All elements to the right of the element, generating a set of numbers.

4. Select a tree from the left and right sets to Saozi the right subtree, together with the root node. Make up a tree.

On Leetcode, the actual execution time is 27ms.

/** * Definition for binary tree * struct TreeNode {* int val; * TreeNode *left; * TreeNode *right; * Tre Enode (int x): Val (x), left (null), right (NULL) {} *};        */class Solution {Public:vector<treenode *> generatetrees (int n) {vector<treenode *> ans;        Generatetrees (ans, 1, n+1);    return ans; } void Generatetrees (Vector<treenode *> &ans, int start, int stop) {for (int i=start; i<stop;            i++) {Vector<treenode *> lefts;            Generatetrees (lefts, start, i);                for (int j=0; j<lefts.size (); j + +) {Vector<treenode *> rights;                Generatetrees (Rights, i+1, stop);                    for (int k=0; k<rights.size (); k++) {TreeNode *root = new TreeNode (i);                    Root->left = Lefts[j];                    Root->right = Rights[k];                Ans.push_back (root);          }            }        }      if (Ans.empty ()) ans.push_back (0); }};

Unique Binary Search Trees II--Leetcode