leetcode 91. Decode Ways

Posted 2022-12-03 zhangwj0101

Question

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

‘A’ -> 1
‘B’ -> 2
…
‘Z’ -> 26
Given an encoded message containing digits, determine the total number of ways to decode it.

For example,
Given encoded message “12”, it could be decoded as “AB” (1 2) or “L” (12).

The number of ways decoding “12” is 2.

Code

public int numDecodings(String s) 
        if (s == null || s.length() == 0) 
            return 0;
        
        int[] nums = new int[s.length() + 1];
        nums[0] = 1;
        nums[1] = s.charAt(0) != '0' ? 1 : 0;
        for (int i = 2; i <= s.length(); i++) 
            if (s.charAt(i - 1) != '0') 
                nums[i] += nums[i - 1];
            
            int twoDigits = (s.charAt(i - 2) - '0') * 10 + s.charAt(i - 1) - '0';
            if (twoDigits >= 10 && twoDigits <= 26) 
                nums[i] += nums[i - 2];
            
        
        return nums[s.length()];