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95.Unique-Binary-Search-Trees-II

95. Unique Binary Search Trees II

题目地址

题目描述

Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1 ... n.
​
Example:
Input: 3
Output:
[
[1,null,3,2],
[3,2,null,1],
[3,1,null,null,2],
[2,1,3],
[1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST's shown below:
​
1 3 3 2 1
\ / / / \ \
3 2 1 1 3 2
/ / \ \
2 1 2 3

代码

Approach #1 Recursion

/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<TreeNode> generateTrees(int n) {
if (n = 0) {
return new LinkedList<TreeNode>();
}
return generate_trees(1, n);
}
​
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;
}
​
for (int i = start; i <= end; i++) {
LinkedList<TreeNode> left_trees = generate_trees(start, i - 1);
LinkedList<TreeNode> right_trees = generate_trees(i + 1, end);
​
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;
}
​
}