java: Created distinct subsequences

Author: rixant

java/115-Distinct-Subsequences.java

@@ -0,0 +1,43 @@
+// Dynammic Programming - Memoization
+// Time Complexity O(s * t) | Space Complexity O(s * t)
+class Solution {
+    public int numDistinct(String s, String t) {
+        
+        int n = s.length() + 1;
+        int m = t.length() + 1;
+        int[][] memo = new int[n][m];
+        
+        for (int[] row : memo){
+				Arrays.fill(row, -1);
+        }
+        
+        return recursion(s, t, 0, 0, memo);
+        
+    }
+    
+    
+    public int recursion(String s, String t, int sIdx, int tIdx, int[][] memo){
+        
+        if (memo[sIdx][tIdx] != -1){
+            return memo[sIdx][tIdx];
+        }
+            
+        if (tIdx >= t.length()){
+            return 1;
+        }
+        
+        if (sIdx >= s.length()){
+            return 0;
+        }
+        
+        if (t.charAt(tIdx) == s.charAt(sIdx)){
+            memo[sIdx][tIdx] = recursion(s, t, sIdx + 1, tIdx + 1, memo) + recursion(s, t, sIdx + 1, tIdx, memo);
+            return memo[sIdx][tIdx];   
+        }
+        
+        memo[sIdx][tIdx] = recursion(s, t, sIdx + 1, tIdx, memo);
+        return memo[sIdx][tIdx];
+        
+    }
+    
+}