Unique Binary Search Trees II

Author: mengli

UniqueBinarySearchTreesII.java

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+/**
+ * Given n, generate all structurally unique BST's (binary search trees) that store values 1...n.
+ *
+ * For example,
+ * Given n = 3, your program should return all 5 unique BST's shown below.
+ *
+ *  1         3     3      2      1
+ *   \       /     /      / \      \
+ *    3     2     1      1   3      2
+ *   /     /       \                 \
+ *  2     1         2                 3
+ */
+
+import java.util.ArrayList;
+
+public class UniqueBinarySearchTreesII {
+	public ArrayList<TreeNode> generateTrees(int n) {
+		return buildBST(1, n);
+	}
+
+	private ArrayList<TreeNode> buildBST(int min, int max) {
+		ArrayList<TreeNode> ret = new ArrayList<TreeNode>();
+		if (min > max) {
+			ret.add(null);
+			return ret;
+		}
+		if (min == max) {
+			ret.add(new TreeNode(min));
+			return ret;
+		}
+		for (int i = min; i <= max; i++) {
+			ArrayList<TreeNode> leftTrees = buildBST(min, i - 1);
+			ArrayList<TreeNode> rightTrees = buildBST(i + 1, max);
+			for (TreeNode l : leftTrees) {
+				for (TreeNode r : rightTrees) {
+					TreeNode root = new TreeNode(i);
+					root.left = l;
+					root.right = r;
+					ret.add(root);
+				}
+			}
+		}
+		return ret;
+	}
+}