Decode Ways

Problem

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).

Optimal Solution

Complexity

• Time: O(n)
• Space: O(n)

public int numDecodings(String s) {
    if(s == null || s.length() == 0) {
        return 0;
    }
    int n = s.length();
    int[] dp = new int[n+1];
    dp[0] = 1;
    dp[1] = s.charAt(0) != '0' ? 1 : 0;
    for(int i = 2; i <= n; i++) {
        int first = Integer.valueOf(s.substring(i-1, i));
        int second = Integer.valueOf(s.substring(i-2, i));
        if(first >= 1 && first <= 9) {
           dp[i] += dp[i-1];  
        }
        if(second >= 10 && second <= 26) {
            dp[i] += dp[i-2];
        }
    }
    return dp[n];
}