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
Solution:
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<TreeNode> generateTrees(int n) {
if(n<1) return new ArrayList<>();
return generateTrees(1,n);
}
public List<TreeNode> generateTrees(int start, int end) {
List<TreeNode> ans = new ArrayList<>();
if(start>end){
ans.add(null);
return ans;
}
for(int i=start; i<=end; i++){
List<TreeNode> left = generateTrees(start,i-1);
List<TreeNode> right = generateTrees(i+1,end);
for(TreeNode l : left){
for(TreeNode r : right){
TreeNode root = new TreeNode(i);
root.left = l;
root.right = r;
ans.add(root);
}
}
}
return ans;
}
}