Add 77_Combinations.java

Author: Sean Prashad

Backtracking/77_Combinations.java

@@ -0,0 +1,24 @@
+class Solution {
+    public List<List<Integer>> combine(int n, int k) {
+        if (n == 0 || k > n) {
+            return Collections.emptyList();
+        }
+
+        List<List<Integer>> result = new ArrayList<>();
+        helper(n, k, 1, result, new ArrayList<>());
+        return result;
+    }
+
+    private void helper(int end, int numbersLeft, int start, List<List<Integer>> result, List<Integer> temp) {
+        if (numbersLeft == 0) {
+            result.add(new ArrayList<>(temp));
+            return;
+        }
+
+        for (int i = start; i <= end; i++) {
+            temp.add(i);
+            helper(end, numbersLeft - 1, i + 1, result, temp);
+            temp.remove(temp.size() - 1);
+        }
+    }
+}