DecodeWays.java

package leetcode;

/**
 * @author eko
 * @date 2018/10/27 2:50 PM
 *
 * A message containing letters from A-Z is being encoded to numbers using the following mapping:
 *
 * 'A' -> 1
 * 'B' -> 2
 * ...
 * 'Z' -> 26
 * Given a non-empty string containing only digits, determine the total number of ways to decode it.
 *
 * Example 1:
 *
 * Input: "12"
 * Output: 2
 * Explanation: It could be decoded as "AB" (1 2) or "L" (12).
 * Example 2:
 *
 * Input: "226"
 * Output: 3
 * Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
 */
public class DecodeWays {
    public static void main(String[] args) {
        String s = "226";
        int res = new DecodeWays().numDecodings(s);
        System.out.println(res);
    }

    public int numDecodings(String s) {
        if (s==null||s.length()==0) {
            return 0;
        }
        int n = s.length();
        int[] dp = new int[n+1];
        dp[0] = 1;
        dp[1] = s.charAt(0) == '0'?0:1;
        for (int i = 2; i <= n ; i++) {
            Integer first = Integer.valueOf(s.substring(i-1, i));
            Integer second = Integer.valueOf(s.substring(i-2, i));
            if (first >= 0 && first <= 9) {
                dp[i] += dp[i-1];
            }
            if (second >= 10 && second <= 26) {
                dp[i] += dp[i-2];
            }
        }

        return dp[n];
    }
}