All Possible Full Binary Trees
给一个数字n, 求n个nodes组成的二叉树.
这里注意几个corn cases, 比如0,1,2
/**
* 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 {
List<TreeNode>[] lists = new List[21];
public List<TreeNode> allPossibleFBT(int n) {
List<TreeNode> res = new LinkedList();
if(n <= 0)
return res;
if(n == 1){
res.add(new TreeNode(0));
return res;
}
if(n % 2 != 1){
return res;
}
if(lists[n] != null){
return lists[n];
}
for(int i = 0; i < n; i++) {
int j = n - i - 1;
for (TreeNode left: allPossibleFBT(i))
for (TreeNode right: allPossibleFBT(j)) {
TreeNode node = new TreeNode(0);
node.left = left;
node.right = right;
res.add(node);
}
}
lists[n] = res;
return res;
}
}