tree

Author: yuzhangcmu

tree/NumTrees.java

@@ -1,7 +1,7 @@
 package Algorithms.tree;
 
 public class NumTrees {
-    public int numTrees(int n) {
+    public int numTrees1(int n) {
         // cnt[n] = cnt[0]cnt[n - 1] + cnt[1]cnt[n - 2] ... cnt[n - 1]cnt[0]
         // cnt[n-1] = cnt[0]cnt[n - 2] + cnt[1]cnt[n - 3] ... cnt[n - 2]cnt[0]
         // For example:
@@ -26,4 +26,19 @@ public int numTrees(int n) {
 
         return cnt[n];
     }
-}
+    
+    public int numTrees(int n) {
+        if (n == 0) {
+            return 1;
+        }
+        
+        // Get the results of all the trees which 
+        // has the root from 1 to n;
+        int num = 0;
+        for (int i = 0; i <= n - 1; i++) {
+            num += numTrees(i) * numTrees(n - 1 - i);
+        }
+        
+        return num;
+    }
+}