91.decode-ways.java

/*
 * @lc app=leetcode id=91 lang=java
 *
 * [91] Decode Ways
 */

// @lc code=start
class Solution {
    public int numDecodings(String s) {
        //dp[i] = dp[i-1] + dp[i-2]
        if (s == null || s.length() == 0) {
            return 0;
        }

        int[] dp = new int[s.length() + 1];
        dp[0] = 1;
        dp[1] = s.charAt(0) != '0' ? 1 : 0; 

        for (int i = 2; i <= s.length(); i++) {
            int first = Integer.valueOf(s.substring(i - 1, i));
            int second = Integer.valueOf(s.substring(i - 2, i));

            if (first >= 1 && first <= 9) {
                dp[i] += dp[i - 1];
            }
            if (second >= 10 && second <= 26) {
                dp[i] += dp[i - 2];
            }
        }
        return dp[s.length()];
    }
}
// @lc code=end