Create 095. Unique Binary Search Trees II.java

Author: futurus

095. Unique Binary Search Trees II.java

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+// https://leetcode.com/problems/unique-binary-search-trees-ii
+
+/**
+ * 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> generate(int first, int last) {
+        List<TreeNode> list = new ArrayList<>();
+        
+        if (first > last) {
+            list.add(null);
+        } else if (first == last) {
+            list.add(new TreeNode(first));
+        } else {
+            List<TreeNode> left = new ArrayList<>();
+            List<TreeNode> right = new ArrayList<>();
+            
+            for (int i = first; i <= last; i++) {
+                left = generate(first, i - 1);
+                right = generate(i + 1, last);
+                
+                for (TreeNode l : left) {
+                    for (TreeNode r : right) {
+                        TreeNode root = new TreeNode(i);
+                        root.left = l;
+                        root.right = r;
+                        list.add(root);
+                    }
+                }
+            }
+        }
+        return list;
+    }
+    
+    public List<TreeNode> generateTrees(int n) {
+        if (n == 0) return new ArrayList<TreeNode>();
+        
+        return generate(1, n);
+    }
+}