Create CombinationSumIII.java

Author: 17tanya

Backtracking/CombinationSumIII.java

@@ -0,0 +1,36 @@
+//LeetCode 216. Combination Sum III
+//Question - https://leetcode.com/problems/combination-sum-iii/
+
+class Solution {
+    //k ==> size of each combination
+    //n ==> target sum of each combination
+    //numbers from [1..9] can be used only once.
+    
+    public List<List<Integer>> combinationSum3(int k, int n) {
+        List<List<Integer>> res = new ArrayList<>();
+        helper(k, n, 1, 0, new ArrayList<>(), res);
+        
+        return res;
+    }
+    
+    public void helper(int k, int n, int num, int sum, List<Integer> l, List<List<Integer>> res){
+        //Check for the base condition
+        if(l.size() == k && sum == n){
+            res.add(new ArrayList<>(l));
+            return;
+        }
+        
+        //to avoid useless recursion
+        else if(l.size() == k || sum > n) return;
+        
+        for(int i = num ; i <= 9 ; i++){
+            //make a decision
+            l.add(i);
+            
+            helper(k, n, i + 1, sum + i, l, res);
+            
+            //backtrack
+            l.remove(l.size()-1);
+        }
+    }
+}