Leetcode: 95. Unique Binary Search Trees II

Description

Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1...n.

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

思路

回溯

对每一个点，递归生成它的左子树和右子树

代码

/**
* 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*> generateTrees(int n) {
vector<TreeNode*> res;
if(n == 0) return res;
res = helper(1, n);

return res;
}

vector<TreeNode*> helper(int start, int end){
vector<TreeNode*> res;
if(start > end){
res.push_back(NULL);
}
else if(start == end){
TreeNode *tmp = new TreeNode(start);
res.push_back(tmp);
}
else{
for(int i = start; i <= end; ++i){
vector<TreeNode*> left = helper(start, i - 1);
vector<TreeNode*> right = helper(i + 1, end);

for(int j = 0; j < left.size(); ++j){
for(int k = 0; k < right.size(); ++k){
TreeNode *root = new TreeNode(i);
root->left = left[j];
root->right = right[k];
res.push_back(root);
}
}
}
}

return res;
}
};