'A' -> 1
'B' -> 2
...
'Z' -> 26
Input: "12"
Output: 2
Explanation:
It could be decoded as "AB" (1 2) or "L" (12).
Input: "226"
Output: 3
Explanation:
It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
class Solution {
public int numDecodings(String s) {
if (s == null || 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 oneDigit = s.charAt(i - 1) - '0';
int twoDigits = (s.charAt(i - 2) - '0') * 10 + oneDigit;
if (oneDigit >= 1 && oneDigit <= 9) {
dp[i] += dp[i - 1];
}
if (twoDigits >= 10 && twoDigits <= 26) {
dp[i] += dp[i - 2];
}
}
return dp[s.length()];
}
}