LeetCode 95. Unique Binary Search Trees II

Given an integer 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

分析

这道题的思路一开始想错了，参考了discussion,当n=m时的结果很容易想到应该是在n=m-1时的结果上得到的，相当于在n=m-1的各棵树插入n这个节点，分为插在根节点上和右子树上两种情况。

/**
 * 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:
        TreeNode* clone(TreeNode* root){
            if(root == nullptr)
                return nullptr;
            TreeNode* newroot = new TreeNode(root->val);
            newroot->left = clone(root->left);
            newroot->right = clone(root->right);
            return newroot;
        }
        vector<TreeNode *> generateTrees(int n) {
            vector<TreeNode*> r;
            if(n==0) return r;
            vector<TreeNode *> res(1,nullptr);
            for(int i = 1; i <= n; i++){
                vector<TreeNode *> tmp;
                for(int j = 0; j<res.size();j++){
                    TreeNode* oldroot = res[j];
                    TreeNode* root = new TreeNode(i);
                    TreeNode* target = clone(oldroot);
                    root->left = target;
                    tmp.push_back(root);
                    if(oldroot!=nullptr){
                        TreeNode* tmpold = oldroot;
                        while(tmpold->right!=nullptr){
                            TreeNode* nonroot = new TreeNode(i);
                            TreeNode *tright = tmpold->right;
                            tmpold->right = nonroot;
                            nonroot->left = tright;
                            TreeNode *target = clone(oldroot);
                            tmp.push_back(target);
                            tmpold->right = tright;
                            tmpold = tmpold->right;
                        }
                        tmpold->right = new TreeNode(i);
                        TreeNode *target = clone(oldroot);
                        tmp.push_back(target);
                        tmpold->right = nullptr;
                    }
                }
                res=tmp;
            }
            return res;
        }
    };