Leetcode no. 91

91. 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.

public class Solution {
public int numDecodings(String s) {
if (s.length() ==0 || s.charAt(0)== '0') return 0;
int prev= 1, pprev= 1;
for (int i = 1; i < s.length(); i++) {
if (s.charAt(i)=='0') prev=0;
if (s.charAt(i-1)=='1' || (s.charAt(i-1)=='2' && s.charAt(i)-'0'<=6)) {
int tmp = prev;
prev+= pprev;
pprev= tmp;
} else pprev= prev;
}
return prev;
}
}