1. 不同的二叉搜索树（中等）
二叉树算法的关键就在于明确根节点需要做什么，其实 BST 作为一种特殊的二叉树，核心思路也是一样的。
地址: https://leetcode-cn.com/problems/unique-binary-search-trees/
2021/12/04
做题反思：新建数组
class Solution {
public int numTrees(int n) {
memo = new int[n + 1][n + 1];
return count(1, n);
}
int[][] memo;
int count(int lo, int hi) {
if (lo > hi) {
return 1;
}
if (memo[lo][hi] != 0) {
return memo[lo][hi];
}
int res = 0;
for (int i = lo; i <= hi; i++) {
int left = count(lo, i - 1);
int right = count(i + 1, hi);
res += left * right;
}
memo[lo][hi] = res;
return res;
}
}
2. 不同的二叉搜索树 II （中等）
地址: https://leetcode-cn.com/problems/unique-binary-search-trees-ii/
2021/12/02
做题反思：res的位置
class Solution {
public List<TreeNode> generateTrees(int n) {
if (n == 0) {
return new LinkedList<>();
}
return build(1, n);
}
List<TreeNode> build(int lo, int hi) {
List<TreeNode> res = new LinkedList<>();
if (lo > hi) {
res.add(null);
return res;
}
for (int i = lo; i <= hi; i++) {
List<TreeNode> leftTree = build(lo, i - 1);
List<TreeNode> rightTree = build(i + 1, hi);
for (TreeNode left : leftTree) {
for (TreeNode right : rightTree) {
TreeNode root = new TreeNode(i);
root.left = left;
root.right = right;
res.add(root);
}
}
}
return res;
}
}
