Create 0077-combinations.java

Author: sujal-goswami

Diff for: java/0077-combinations.java

@@ -0,0 +1,21 @@
+class Solution {
+    List<List<Integer>> res;
+    public List<List<Integer>> combine(int n, int k) {
+        res = new ArrayList<>();
+        backtrack(1, new ArrayList<Integer>(), n, k);
+        return res;
+    }
+
+    private void backtrack(int start, ArrayList<Integer> comb, int n, int k) {
+        if (comb.size() == k){
+            res.add(new ArrayList<>(comb));
+            return;
+        }
+
+        for (int i = start; i <= n; i++) {
+            comb.add(i);
+            backtrack(i+1, comb, n, k);
+            comb.remove((Integer) i);
+        }
+    }
+}