add solution of decode-ways

Author: jinhyungrhee

decode-ways/jinhyungrhee.java

@@ -0,0 +1,33 @@
+import java.util.*;
+class Solution {
+
+    public List<Integer> nums;
+    public int numDecodings(String s) {
+        nums = new ArrayList<>();
+        for (int i = 0; i < s.length(); i++) {
+            nums.add(s.charAt(i) - 48);
+        }
+
+        int[] memo = new int[nums.size() + 1];
+        Arrays.fill(memo, -1);
+        return dfs(0, memo);
+    }
+
+    public int dfs(int start, int[] memo) {
+
+        if (start == nums.size()) return 1;
+
+        if (memo[start] != -1) {
+            return memo[start];
+        }
+        if (nums.get(start) == 0) {
+            memo[start] = 0;
+        }
+        else if (start + 1 < nums.size() && nums.get(start) * 10 + nums.get(start + 1) < 27) {
+            memo[start] = dfs(start + 1, memo) + dfs(start + 2, memo);
+        } else {
+            memo[start] = dfs(start + 1, memo);
+        }
+        return memo[start];
+    }
+}