Add num decoding ways solution

Author: sudheerj

src/java1/algorithms/dynamicProgramming/NumDecodingWays.java

@@ -0,0 +1,59 @@
+package java1.algorithms.dynamicProgramming;
+
+public class NumDecodingWays {
+    // TC:O(n) SC:O(1)
+    private static int numDecodingWays1(String str) {
+        if((str.length() == 0) || (str.charAt(0) == '0')) return 0;
+
+        if(str.length() == 1) return 1;
+
+        int prevCount1 = 1, prevCount2 = 1;
+
+        for(int i=2; i<=str.length(); i++) {
+            int oneDigit = Integer.valueOf(str.substring(i-1, i));
+            int twoDigit = Integer.valueOf(str.substring(i-2, i));
+            int count = 0;
+
+            if(oneDigit >=1) {
+                count += prevCount2;
+            }
+            if(twoDigit >=10 && twoDigit <=26) {
+                count += prevCount1;
+            }
+            prevCount1 = prevCount2;
+            prevCount2 = count;
+
+        }
+        return prevCount2;
+    }
+
+    // TC:O(n) SC:O(n)
+    private static int numDecodingWays2(String str) {
+        if(str.charAt(0) == '0') return 0;
+        int[] dp = new int[str.length()+1];
+        dp[0] = 1;
+        dp[1] = str.charAt(0) == '0' ? 0 : 1;
+
+        for(int i=2; i<=str.length(); i++) {
+            int oneDigit = Integer.valueOf(str.substring(i-1, i));
+            int twoDigit = Integer.valueOf(str.substring(i-2, i));
+
+            if(oneDigit >=1) {
+                dp[i] += dp[i-1];
+            }
+            if(twoDigit >= 10 && twoDigit <=26) {
+                dp[i] += dp[i-2];
+            }
+        }
+        return dp[str.length()];
+    }
+
+    public static void main(String[] args) {
+        String str1 = "221";
+        String str2 = "06";
+        System.out.println(numDecodingWays1(str1));
+        System.out.println(numDecodingWays1(str2));
+        System.out.println(numDecodingWays2(str1));
+        System.out.println(numDecodingWays2(str2));
+    }
+}

src/javascript/algorithms/dynamicProgramming/numDecodingWays.js

@@ -0,0 +1,53 @@
+// TC:O(n) SC:O(1)
+function numDecodingWays1(str){
+    if(str.length === 0 || str.charAt(0) === '0') return 0;
+    if(str.length === 1) return 1;
+
+    let prevCount1 = prevCount2 = 1;
+    for(let i=2; i<= str.length; i++) {
+        let oneDigit = Number.parseInt(str.substr(i-1, i));
+        let twoDigit = Number.parseInt(str.substr(i-2, i));
+        let count = 0;
+
+        if(oneDigit >=1) {
+            count += prevCount2;
+        }
+
+        if(twoDigit >=10 && twoDigit <=26) {
+            count += prevCount1;
+        }
+
+        prevCount1 = prevCount2;
+        prevCount2 = count;
+    }
+    return prevCount2;
+}
+
+ // TC:O(n) SC:O(n)
+ function numDecodingWays2(str){
+    let dp = new Array(str.length+1).fill(0);
+
+    dp[0] = 1;
+    dp[1] = str.charAt(0) === '0' ? 0 : 1;
+
+    for(let i=2; i<= str.length; i++) {
+        let oneDigit = Number.parseInt(str.substr(i-1, i));
+        let twoDigit = Number.parseInt(str.substr(i-2, i));
+
+        if(oneDigit >=1) {
+            dp[i] += dp[i-1];
+        }
+
+        if(twoDigit >=10 && twoDigit <=26) {
+            dp[i] += dp[i-2];
+        }
+    }
+    return dp[str.length];
+ }
+
+ let str1 = "221";
+ let str2 = "06";
+ console.log(numDecodingWays1(str1));
+ console.log(numDecodingWays1(str2));
+ console.log(numDecodingWays2(str1));
+ console.log(numDecodingWays2(str2));