Thinking process

Find the pattern

But be careful with 0

Mistakes

Instead of thinking of all possibilities, why not only thinking of the conditions that you need to do sth on it.

public class Solution {
    public int numDecodings(String s) {
        if (s.length() == 0 || s.charAt(0) == '0') return 0;
        int[] dp = new int[s.length() + 1];
        dp[0] = 1;
        dp[1] = 1;
        for (int i = 2; i <= s.length(); ++i) {
            int single = Integer.valueOf(s.substring(i - 1, i));
            if (single >= 1 && single <= 9) {
                dp[i] += dp[i - 1];
            }

            int twoNums = Integer.valueOf(s.substring(i - 2, i));
            if (twoNums >= 10 && twoNums <= 26) {
                dp[i] += dp[i - 2];
            }
        }

        return dp[s.length()];
    }
}