Create 0377-combination-sum-iv.java

java/0377-combination-sum-iv.java

@@ -0,0 +1,51 @@
+class Solution {
+
+  /*       Tabulation Method
+      --------------------------
+      T = Target, N = nums.length
+      Time complexity: O(T⋅N)
+      Space complexity: O(T)
+  */
+
+    public int combinationSum4(int[] nums, int target) {
+        int[] dp = new int[target+1];
+        dp[0] = 1;
+
+        for(int currSum = 1; currSum < dp.length; currSum++){
+            for(int no : nums){
+                if(currSum - no >= 0){
+                    dp[currSum] += dp[currSum - no];
+                }
+            }
+        }
+        return dp[target];
+    }
+
+/*        Memoization Method
+      --------------------------
+      T = Target, N = nums.length
+      Time complexity: O(T⋅N)
+      Space complexity: O(T) + Recursive Stack
+*/
+
+    public int combinationSum4(int[] nums, int target) {
+        HashMap<Integer, Integer> memo = new HashMap<>();
+        return helper(nums, target, memo);
+    }
+
+    private int helper(int[] nums, int t, HashMap<Integer, Integer> memo){
+        if(t == 0)
+            return 1;
+        if(t < 0)
+            return 0;
+        if(memo.containsKey(t))
+            return memo.get(t);
+
+        int count = 0;
+        for(int no : nums){
+            count += helper(nums, t - no);
+        }
+        memo.put(t, count);
+        return count;        
+    }
+}