link
Approach: Recursion1
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38/**
* 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 {
private List<TreeNode> genTreeList(int start,int end){
List<TreeNode> ls=new ArrayList<>();
if(start>end) ls.add(null);
for(int i=start;i<=end;i++){
List<TreeNode> leftTreeList=genTreeList(start,i-1);
List<TreeNode> rightTreeList=genTreeList(i+1,end);
for(TreeNode left:leftTreeList){
for(TreeNode right:rightTreeList){
TreeNode root=new TreeNode(i);
root.left=left;
root.right=right;
ls.add(root);
}
}
}
return ls;
}
public List<TreeNode> generateTrees(int n) {
if(n==0) return new ArrayList<>();
else return genTreeList(1,n);
}
}