[leetcode-95]Unique Binary Search Trees II(c++)

问题描述：

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

分析：这道题的思想与上题是一样的。区别在于这题要求把满足条件的都列出来，这样就不能再DP了，只能用递归的算法。

代码如下：28ms

[code]/**
 * 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:
    vector<TreeNode*> genearateTree(int start, int end) {
        vector<TreeNode*> res;
        vector<TreeNode*> tmpVec;
        TreeNode *cur;
        if (start > end) {
            res.push_back(NULL);
            return res;
        }
        for (int i = start; i <= end; i++) {
            vector<TreeNode*> left = genearateTree(start, i-1);
            vector<TreeNode*> right = genearateTree(i + 1, end);
            for (int j1 = 0; j1 < left.size(); j1++) {
                for (int j2 = 0; j2 < right.size(); j2++) {
                    TreeNode* cur = new TreeNode(i);
                    cur->left = left[j1];
                    cur->right = right[j2];
                    res.push_back(cur);
                }
            }
        }
        return res;
    }
    vector<TreeNode*> generateTrees(int n) {
        return genearateTree(1,n);
    }
};