Algorithms—95.Unique Binary Search Trees II

思路：再上一题的基础上继续使用动态规划。

/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<TreeNode> generateTrees(int n) {
return f(1,n,new HashMap<String, List<TreeNode>>());
}
public List<TreeNode> f(int s,int e,Map<String,List<TreeNode>> map) {
if (map.get(""+s+","+e)==null) {
List<TreeNode> ans=new ArrayList<TreeNode>();
if (s>e) {
ans.add(null);
}else if (s==e) {
ans.add(new TreeNode(s));
}else {
for (int i = s; i <=e; i++) {
List<TreeNode> left=f(s,i-1,map);
List<TreeNode> right=f(i+1,e,map);
for (int j = 0; j < left.size(); j++) {
for (int k = 0; k < right.size(); k++) {
TreeNode node=new TreeNode(i);
node.left=left.get(j);
node.right=right.get(k);
ans.add(node);
}
}
}
}
map.put(""+s+","+e, ans);
return ans;
}
return map.get(""+s+","+e);
}
}