### 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.

``````/**
* Created by gzdaijie on 16/6/5
* 0,30,40,50... => 0
* 10,20 => f(n) = f(n-2)
* <=9 || > 26 => f(n) = f(n -1)
* 11 - 26 => f(n) = f(n-1) + f(n - 2)
* 动态规划,时间复杂度O(N), 空间复杂度O(N)
*/
public class Solution {
public int numDecodings(String s) {
int len = s.length();
if (len == 0 || s.charAt(0) == '0') return 0;

int[] count = new int[len + 1];
count[0] = count[1] = 1;
for (int i = 1; i < len; i++) {
int sum = (s.charAt(i - 1) - '0') * 10 + (s.charAt(i) - '0');
if (sum % 10 == 0) {
if(sum == 10 || sum == 20) count[i + 1] = count[i - 1];
else return 0;
}
else if (sum < 10 || sum > 26) count[i + 1] = count[i];
else count[i + 1] = count[i] + count[i - 1];
}
return count[len];
}
}
``````
updated 2016-06-05 15:50:34