print all N choose k combination

Recursion/Combination.java

@@ -0,0 +1,42 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+//N Choose K Combinations
+//        Given two integers n and k, find all the possible unique combinations of k numbers in range 1 to n.
+//        Example One:
+//        Input: 5, 2
+//        Output: [[1, 2], [1, 3], [1, 4], [1, 5], [2, 3], [2, 4], [2, 5], [3, 4], [3, 5], [4, 5]].
+
+//77 leetCode Combination
+public class Combination {
+
+    public static List<String> combination(int n, int k) {
+        char[] slate = new char[k];
+        List<String> result = new ArrayList<>();
+        combinationHelper(n, k, 1, slate, 0, result);
+        return result;
+    }
+
+    private static void combinationHelper(int n, int k, int startNum, char[] slate, int slateCur, List<String> result) {
+        if (slateCur == k) { //backTracking
+            result.add(new String(slate,0,k));
+            return;
+        }
+
+        if (startNum > n) // as we exclude this is a possibility
+            return;
+
+        combinationHelper(n , k, startNum + 1, slate, slateCur, result);
+        slate[slateCur] = (char)('0'+startNum);
+        combinationHelper(n, k, startNum + 1, slate, slateCur+1, result);
+    }
+
+    public static void main(String[] args) {
+        List<String> s = combination(4,2);
+        System.out.println(s);
+    }
+}
+
+
+