LeetCode-M-Unique Binary Search Trees II

##题意

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

解法

实现

/**
* 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*> generateTreesHelper(int start, int end){
vector<TreeNode*> result;
if(start > end) {
result.push_back(NULL);
}else if(start == end){
TreeNode* root = new TreeNode(start);
result.push_back(root);
}else{
for(int i = start; i <= end; ++i){
vector<TreeNode*> left = generateTreesHelper(start,i-1);
vector<TreeNode*> right = generateTreesHelper(i+1,end);
for(int l = 0; l < left.size(); ++l){
for(int r = 0; r < right.size(); ++r){
TreeNode* root = new TreeNode(i);
root->left = left[l];
root->right = right[r];
result.push_back(root);
}
}
}
}
return result;
}
vector<TreeNode*> generateTrees(int n) {
vector<TreeNode*> ivec;
if(n <= 0) return ivec;
ivec = generateTreesHelper(1,n);
return ivec;
}
};