Decode Ways

dynamic programming

public class Solution {
    public int numDecodings(String s) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        int length = s.length();
        if(0 == length || s.charAt(0) == '0')
            return 0;
        int[] dp = new int[length+1];
        dp[0] = 1;
        dp[1] = 1;
        for(int i = 2; i < length+1; i++)
        {
            char curChar = s.charAt(i - 1);
            int curNum = curChar - '0';
            // s[i] is not valid
            if (curNum == 0) {
                String twoNum = s.substring(i - 2, i);
                // s[i-1][i] is valid
                if (Integer.parseInt(twoNum) <= 26
                         && Integer.parseInt(twoNum) >= 10) {
                     dp[i] = dp[i - 2];
                 } else {
                     dp[i] = 0;
                 }
             }
             // s[i] is valid
             else {
                 String twoNum = s.substring(i - 2, i);
                // s[i-1][i] is valid
                if (Integer.parseInt(twoNum) <= 26
                        && Integer.parseInt(twoNum) >= 10) {
                    dp[i] = dp[i - 1] + dp[i - 2];
                } else {
                    dp[i] = dp[i - 1];
                }
            }
        }
        return dp[length];
    }
}