[LeetCode] 91. 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 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.

Solution: Dynamic Programming for better performance and avoid time-limit exception. Iterate the string and use an array to store the current number of decodings. 

class Solution {

    public int numDecodings(String s) {

        if (s == null || s.isEmpty() || s.charAt(0) == '0') return 0;

        if (s.length() == 1) return 1;

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

        dp[0] = 1;

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

            if (s.charAt(i) >= '1') dp[i] += dp[i - 1];

            if (s.charAt(i - 1) == '1' || (s.charAt(i - 1) == '2' && s.charAt(i) <= '6')) {

                if (i < 2) dp[i] += 1;

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

            }

        }

        

        return dp[s.length() - 1];

    }

}