Java for LeetCode 091 Decode Ways

A message containing letters from

A-Z

is being encoded to numbers using the following mapping:

'A' -> 1
'B' -> 2
...
'Z' -> 26

Given an encoded message containing digits, determine the total number of ways to decode it.

For example,

Given encoded message

"12"

,
it could be decoded as

"AB"

(1 2) or

"L"

(12).

The number of ways decoding

"12"

is 2.

解题思路：

dp问题，JAVA实现如下：

static public int numDecodings(String s) {
if (s.length() == 0 || s.charAt(0) == '0')
return 0;
int[] dp = new int[s.length() + 1];
dp[0] = dp[1] = 1;
for (int i = 1; i < s.length(); i++) {
if (s.charAt(i) == '0'&& (s.charAt(i - 1) != '1' && s.charAt(i - 1) != '2'))
return 0;
else if (s.charAt(i) == '0')
dp[i + 1] = dp[i - 1];
else if (s.charAt(i - 1) == '0'||(s.charAt(i-1)-'0')*10+(s.charAt(i)-'0')>26)
dp[i + 1] = dp[i];
else
dp[i + 1] = dp[i - 1] + dp[i];
}
return dp[s.length()];
}