problems: add combination sum IV

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/combination-sum-iv.java

@@ -0,0 +1,35 @@
+import java.util.TreeMap;
+
+class Solution {
+    private TreeMap<Integer, Integer> cache;
+
+    private int combinationSumRec(int[] nums, int target) {
+        if (target < 0) {
+            // Base: Target is below zero
+            return 0;
+        }
+
+        if (target == 0) {
+            // Base: We have found a possible combination
+            return 1;
+        }
+
+        if (cache.containsKey(target)) {
+            // Base: Target is already precomputed
+            return cache.get(target);
+        }
+
+        int combinations = 0;
+        for (int num : nums) {
+            combinations += combinationSumRec(nums, target - num);
+        }
+
+        cache.put(target, combinations);
+        return combinations;
+    }
+
+    public int combinationSum4(int[] nums, int target) {
+        cache = new TreeMap<>();
+        return combinationSumRec(nums, target);
+    }
+}