https://github.com/emahtab/decode-ways

Number of ways to decode
https://github.com/emahtab/decode-ways

dynamic-programming leetcode problem-solving

Last synced: 3 months ago
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Number of ways to decode

• Host: GitHub
• URL: https://github.com/emahtab/decode-ways
• Owner: eMahtab
• Created: 2020-04-16T14:28:13.000Z (about 5 years ago)
• Default Branch: master
• Last Pushed: 2020-04-16T14:37:51.000Z (about 5 years ago)
• Last Synced: 2025-02-02T03:26:11.350Z (5 months ago)
• Topics: dynamic-programming, leetcode, problem-solving
• Homepage:
• Size: 125 KB
• Stars: 1
• Watchers: 1
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md

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README

        
# Decode Ways

## https://leetcode.com/problems/decode-ways

A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1

'B' -> 2

...

'Z' -> 26

Given a non-empty string containing only digits, determine the total number of ways to decode it.

```

Example 1:

Input: "12"

Output: 2

Explanation: It could be decoded as "AB" (1 2) or "L" (12).

Example 2:

Input: "226"

Output: 3

Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).

```

## Approach :

![Decode Ways](decode-ways.PNG)

## Implementation : Time => O(number of characters) , Space => O(number of characters)

```java

class Solution {

    public int numDecodings(String s) {

        if(s == null || s.length() == 0) {

            return 0;

        }

        // DP array to store the subproblem results

        int[] dp = new int[s.length() + 1];

        dp[0] = 1;

        // Ways to decode a string of size 1 is 1. Unless the string is '0'.

        // '0' doesn't have a single digit decode.

        dp[1] = s.charAt(0) == '0' ? 0 : 1;

        for(int i = 2; i < dp.length; i += 1) {

            // Check if successful single digit decode is possible.

            if(s.charAt(i-1) != '0') {

               dp[i] = dp[i-1];  

            }

            // Check if successful two digit decode is possible.

            int twoDigit = Integer.parseInt(s.substring(i-2, i));

            if(twoDigit >= 10 && twoDigit <= 26) {

                dp[i] += dp[i-2];

            }

        }

        return dp[s.length()];

    }

}

```

# References :

1. https://leetcode.com/articles/decode-ways