feat: solve decode ways

Author: GangBean

decode-ways/GangBean.java

@@ -0,0 +1,45 @@
+class Solution {
+    public int numDecodings(String s) {
+        /**
+            1. understanding
+            - number to upper case alphabet mapping code: 1 -> A, ... 26 -> Z
+            - many ways to decode each input string
+            - also there can be no way to decode input string.
+            - answer fits in 32-bit integer.
+
+            2. example
+            - 12: (1, 2), (12)
+            - 226: (2, 2, 6), (2, 26), (22, 6)
+            - 06: (0, x)
+
+            3. strategy
+            - iterate in reverse order,
+            - at index k, dp[k] means the count of decode ways till that index.
+            - dp[k-1] = 0 if num[k] == 0
+            - dp[k-1] = dp[k] + dp[k+1] if 1<= nums[k-1:k] < 27
+            - dp[k-1] = dp[k]
+            - dp[n] = 1 -> assume that first empty input can be decoded in 1 way.
+
+            4. complexity
+            - time: O(N)
+            - space: O(1)
+        */
+        int prev = 1;
+        int current = 1;
+        for (int i = s.length()-1; i >= 0; i--) { // O(1)
+            if (s.charAt(i) == '0') {
+                int tmp = current;
+                current = 0;
+                prev = tmp;
+            } else if ( (i+1) < s.length() && Integer.parseInt(s.substring(i, i+2)) < 27) {
+                int tmp = current;
+                current = prev + current;
+                prev = tmp;
+            } else {
+                prev = current;
+            }
+        }
+        return current;
+    }
+}
+