feat: decode ways

Author: minji-go

decode-ways/minjigo.java

@@ -0,0 +1,28 @@
+/*
+    Problem: https://leetcode.com/problems/decode-ways/
+    Description: Given a string s containing only digits, return the number of ways to decode it
+    Concept: String, Dynamic Programming
+    Time Complexity: O(N), Runtime 1ms
+    Space Complexity: O(N), Memory 42.12MB
+*/
+class Solution {
+    public int numDecodings(String s) {
+        int[] dp = new int[s.length()];
+        if(decode(s.substring(0, 1))) dp[0]=1;
+        if(s.length()>1 && decode(s.substring(1, 2))) dp[1]+=dp[0];
+        if(s.length()>1 && decode(s.substring(0, 2))) dp[1]+=dp[0];
+
+        for(int i=2; i<s.length(); i++){
+            if(decode(s.substring(i,i+1))) dp[i]+=dp[i-1];
+            if(decode(s.substring(i-1,i+1))) dp[i]+=dp[i-2];
+        }
+        return dp[s.length()-1];
+    }
+
+    public boolean decode(String s){
+        int num = Integer.parseInt(s);
+        int numLength = (int) Math.log10(num) + 1;
+        if(num<0 || num>26 || numLength != s.length()) return false;
+        return true;
+    }
+}