# 19.2.24 [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.

Example:

Input: 3
Output:
[
[1,null,3,2],
[3,2,null,1],
[3,1,null,null,2],
[2,1,3],
[1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST's shown below:

1         3     3      2      1
\       /     /      / \      \
3     2     1      1   3      2
/     /       \                 \
2     1         2                 3

## 题解

 1 class Solution {
2 public:
3     vector<TreeNode*> build(int s, int e) {
4         if (s > e)return vector<TreeNode*>();
5         vector<TreeNode*>ans;
6         if (s == e) {
7             TreeNode*tmp = new TreeNode(s);
8             ans.push_back(tmp);
9             return ans;
10         }
11         for (int i = s; i <= e; i++) {
12             vector<TreeNode*>left = build(s, i - 1), right = build(i + 1, e);
13             for(int j=0;j<left.size();j++)
14                 for (int k = 0; k < right.size(); k++) {
15                     TreeNode*tmp = new TreeNode(i);
16                     tmp->left = left[j], tmp->right = right[k];
17                     ans.push_back(tmp);
18                 }
19             if (left.size() == 0 || right.size() == 0) {
20                 for (int j = 0; j < left.size(); j++) {
21                     TreeNode*tmp = new TreeNode(i);
22                     tmp->left = left[j];
23                     ans.push_back(tmp);
24                 }
25                 for (int j = 0; j < right.size(); j++) {
26                     TreeNode*tmp = new TreeNode(i);
27                     tmp->right = right[j];
28                     ans.push_back(tmp);
29                 }
30             }
31         }
32         return ans;
33     }
34     vector<TreeNode*> generateTrees(int n) {
35         return build(1, n);
36     }
37 };
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posted @ 2019-02-24 22:08  TobicYAL  阅读(173)  评论(0编辑  收藏  举报