给定一个整数 n,生成所有由 1 ... n 为节点所组成的二叉搜索树。
示例:
输入: 3
输出:
[
[1,null,3,2],
[3,2,null,1],
[3,1,null,null,2],
[2,1,3],
[1,null,2,null,3]
]
解释:
以上的输出对应以下 5 种不同结构的二叉搜索树:
1 3 3 2 1
\ / / / \ \
3 2 1 1 3 2
/ / \ \
2 1 2 3
思路:枚举每个root和对应的左右子树情况,然后组合即可。
/*** Definition for a binary tree node.* public class TreeNode {* int val;* TreeNode left;* TreeNode right;* TreeNode(int x) { val = x; }* }*/
class Solution {public LinkedList<TreeNode> generate_trees(int start, int end) {LinkedList<TreeNode> all_trees = new LinkedList<TreeNode>();if (start > end) {all_trees.add(null);return all_trees;}//枚举所有rootfor (int i = start; i <= end; i++) {// 枚举左子树所有情况LinkedList<TreeNode> left_trees = generate_trees(start, i - 1);// 枚举右子树所有情况LinkedList<TreeNode> right_trees = generate_trees(i + 1, end);// 连接起来的所有情况for (TreeNode l : left_trees) {for (TreeNode r : right_trees) {TreeNode current_tree = new TreeNode(i);current_tree.left = l;current_tree.right = r;all_trees.add(current_tree);}}}return all_trees;}public List<TreeNode> generateTrees(int n) {if (n == 0) return new LinkedList<TreeNode>();return generate_trees(1, n);}
}