LeetCode: Decode Ways

LeetCode: Decode Ways 

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import static org.junit.Assert.assertSame;
/**
 * Created by hzhou on 5/18/15. [email protected]
 * <p/>
 * A message containing letters from A-Z is being encoded to numbers using the following mapping:
 * <pre>
 * 'A' -> 1
 * 'B' -> 2
 * ...
 * 'Z' -> 26
 * </pre>
 * Given an encoded message containing digits, determine the total number of ways to decode it.
 * <p/>
 * For example, Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).
 * <p/>
 * The number of ways decoding "12" is 2.
 * <p/>
 * http://www.cnblogs.com/springfor/p/3896162.html
 */
public class DecodeWays {
    public int numDecodings(String s) {
        if (s == null || s.isEmpty() || s.equals("0")) {
            return 0;
        }
        int[] map = new int[s.length() + 1];
        map[0] = 1;
        map[1] = isValid(s.substring(0, 1)) ? 1 : 0;
        for (int i = 2; i <= s.length(); i++) {
            if (isValid(s.substring(i - 1, i))) {
                map[i] += map[i - 1];
            }
            if (isValid(s.substring(i - 2, i))) {
                map[i] += map[i - 2];
            }
        }
        return map[s.length()];
    }
    private boolean isValid(String s) {
        if (s == null || s.isEmpty() || s.charAt(0) == '0') {
            return false;
        }
        int value = Integer.valueOf(s);
        return value > 0 && value < 27;
    }
    @Test
    public void test() {
        assertSame(2, numDecodings("12"));
    }
}