题目
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
思路
依次将每个数作为根节点，然后对根节点左边的数和右边的数做递归运算即可
代码
/**
* 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 ==
0)
return new ArrayList<TreeNode>();
return constructTrees(
1,n);
}
public List<TreeNode>
constructTrees(
int left,
int right){
if(left > right){
List<TreeNode> listTrees =
new ArrayList<TreeNode>();
listTrees.add(
null);
return listTrees;
}
List<TreeNode> listTrees =
new ArrayList<TreeNode>();
for(
int i=left;i<=right;i++){
List<TreeNode> listLeftTrees = constructTrees(left,i-
1);
List<TreeNode> listRightTrees = constructTrees(i+
1,right);
for(TreeNode leftNode:listLeftTrees){
for(TreeNode rightNode:listRightTrees){
TreeNode rootNode =
new TreeNode(i);
rootNode.left = leftNode;
rootNode.right = rightNode;
listTrees.add(rootNode);
}
}
}
return listTrees;
}
}
