#268 Decode Ways

Author: forest000014

decode-ways/forest000014.java

@@ -0,0 +1,55 @@
+/*
+# Time Complexity: O(n)
+# Space Complexity: O(n)
+*/
+class Solution {
+
+    private boolean check12(char ch) {
+        return (ch == '1' || ch == '2');
+    }
+
+    private boolean check1(char ch) {
+        return ch == '1';
+    }
+
+    private boolean check2(char ch) {
+        return ch == '2';
+    }
+
+    private boolean check0(char ch) {
+        return ch == '0';
+    }
+
+    private boolean check6(char ch) {
+        return ch <= '6';
+    }
+
+    public int numDecodings(String s) {
+        int n = s.length();
+
+        if (n == 0)
+            return 0;
+
+        int[] dp = new int[n + 1];
+
+        if (check0(s.charAt(0)))
+            return 0;
+
+        dp[0] = 1;
+        dp[1] = 1;
+        if (n == 1)
+            return dp[1];
+
+        for (int i = 1; i < n; i++) {
+            if (check0(s.charAt(i)) && !check12(s.charAt(i - 1)))
+                return 0;
+
+            if (!check0(s.charAt(i)))
+                dp[i + 1] = dp[i];
+
+            if (check1(s.charAt(i - 1)) || (check6(s.charAt(i)) && check2(s.charAt(i - 1))))
+                dp[i + 1] += dp[i - 1];
+        }
+        return dp[n];
+    }
+}