Given an integer n
, return all the structurally unique BST's (binary search trees), which has exactly n
nodes of unique values from 1
to n
. Return the answer in any order.
Example 1:
Input: n = 3
Output: [[1,null,2,null,3],[1,null,3,2],[2,1,3],[3,1,null,null,2],[3,2,null,1]]
Example 2:
Input: n = 1
Output: [[1]]
Constraints:
1 <= n <= 8
复习时不会的地方:返回的树其实就是个list
List<TreeNode> result = new ArrayList<TreeNode>();在子函数中,因为left新生成的树也是子树
就是生成东西的题,用dc
参考:https://leetcode.com/problems/unique-binary-search-trees-ii/discuss/31600/Quite-clean-Java-solution-with-explanation
public class Solution {
public List<TreeNode> generateTrees(int n) {
if(n == 0)
return new ArrayList<TreeNode>();
return generateTrees(1, n);
}
List<TreeNode> generateTrees(int start, int end) {
List<TreeNode> result = new ArrayList<TreeNode>();
if(start > end) {
result.add(null);
return result;
}
for(int i = start; i <= end; i++) {
List<TreeNode> leftSubTrees = generateTrees(start, i - 1);
List<TreeNode> rightSubTrees = generateTrees(i + 1, end);
//左右每个点都遍历到
for(TreeNode left : leftSubTrees) {
for(TreeNode right : rightSubTrees) {
TreeNode root = new TreeNode(i);
root.left = left;
root.right = right;
result.add(root);
}
}
}
return result;
}
}