Add solution.

Author: yuduozhou

Combination.java

@@ -0,0 +1,30 @@
+public class Solution {
+    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
+        // Start typing your Java solution below
+        // DO NOT write main() function
+        return findCombine(n, k);
+        
+    }
+    
+    public ArrayList<ArrayList<Integer>> findCombine(int n, int k){
+        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
+        if (k == 1){
+            for (int i = 1; i <=n; i++){
+                ArrayList<Integer> temp = new ArrayList<Integer>();
+                temp.add(i);
+                result.add(temp);
+            }
+            return result;
+        }
+        else{
+            for (int m = n; m >0; m--){
+                for (ArrayList<Integer> al : findCombine(m - 1, k - 1)){
+                    al.add(m);
+                    result.add(al);
+                }
+            }
+            return result;
+        }
+        
+    }
+}