95. 不同的二叉搜索树 II
题目来源
95. 不同的二叉搜索树 II
题目描述
题目解析
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<TreeNode> generateTrees(int n) {
if(n == 0){
return new LinkedList<>();
}
return helper(1, n);
}
private List<TreeNode> helper(int start, int end){
List<TreeNode> list = new LinkedList<>();
if (start > end){
list.add(null); // 必须,否则无法进入TreeNode l : left等循环
return list;
}
for (int i = start; i <= end; i++){
List<TreeNode> left = helper(start, i - 1);
List<TreeNode> right = helper(i + 1, end);
for (TreeNode l : left){
for (TreeNode r: right){
TreeNode root = new TreeNode(i);
root.left = l;
root.right = r;
list.add(root);
}
}
}
return list;
}
}
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