Completed CombinationSum

src/CombinationSum.java

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+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+/*
+ * TIME COMPLEXITY (both approaches):
+ * Exponential in the worst case (number of combinations grows quickly).
+ * Rough intuition: explore many branches until target is reduced to 0.
+ *
+ * SPACE COMPLEXITY:
+ * - Recursion depth: O(target / minCandidate)
+ * - Path size: O(target / minCandidate)
+ * - Output storage: can be large depending on number of valid combinations.
+ */
+public class CombinationSum {
+
+    // Stores all valid combinations
+    private List<List<Integer>> result;
+
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        this.result = new ArrayList<>();
+        // Pick ONE approach
+        forwardTrackingHelper(candidates, target, 0, new ArrayList<>()); // for-loop based
+        //recursionChooseSkipHelper(candidates, target, 0, new ArrayList<>()); // recursion choose/skip based
+
+        return result;
+    }
+    //1) FOR-LOOP BACKTRACKING
+
+    private void forwardTrackingHelper(int[] candidates, int target, int start, List<Integer> path) {
+        // Found exact target
+        if (target < 0 || start == candidates.length) return;
+        if (target == 0) {
+            result.add(new ArrayList<>(path));
+            return;
+        }
+
+        for (int i = start; i < candidates.length; i++) {
+            int val = candidates[i];
+            // Choose
+            List<Integer> newPath = new ArrayList<>(path);
+            newPath.add(val);
+            // Reuse allowed => pass i (not i+1)
+            forwardTrackingHelper(candidates, target - val, i, newPath);
+
+        }
+    }
+    // 2) RECURSION (CHOOSE / SKIP)
+    private void recursionChooseSkipHelper(int[] candidates, int target, int start, List<Integer> path) {
+        // Success
+        if (target == 0) {
+            result.add(new ArrayList<>(path));
+            return;
+        }
+
+        // Fail / stop
+        if (target < 0 || start == candidates.length) {
+            return;
+        }
+
+        // SKIP current candidate => move to next index
+        recursionChooseSkipHelper(candidates, target, start + 1, path);
+
+        // CHOOSE current candidate => stay on same index (reuse allowed)
+        path.add(candidates[start]);
+        recursionChooseSkipHelper(candidates, target - candidates[start], start, path);
+        path.remove(path.size() - 1); // backtrack
+    }
+    // TESTS
+    public static void main(String[] args) {
+        CombinationSum sol = new CombinationSum();
+
+        int[] candidates1 = {2, 3, 6, 7};
+        int target1 = 7;
+        System.out.println("Input: " + Arrays.toString(candidates1) + ", target=" + target1);
+        System.out.println("Output: " + sol.combinationSum(candidates1, target1));
+
+        int[] candidates2 = {2, 3, 5};
+        int target2 = 8;
+        System.out.println("\nInput: " + Arrays.toString(candidates2) + ", target=" + target2);
+        System.out.println("Output: " + sol.combinationSum(candidates2, target2));
+
+        int[] candidates3 = {4, 5};
+        int target3 = 3;
+        System.out.println("\nInput: " + Arrays.toString(candidates3) + ", target=" + target3);
+        System.out.println("Output: " + sol.combinationSum(candidates3, target3));
+    }
+
+}