[leetcode]91. Decode Ways

题目：

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.

分析：动态规划，考虑当前位一位是否成立，当前位与前一位构成的两位数是否成立。

代码：

class Solution {
public:
    bool check(char c){
        if(c-'0'>0){
            return true;
        }
        return false;
    }
    bool check(char c1,char c2){
        if((c1=='1')||(c1=='2'&&c2<'7')){
            return true;
        }
        return false;
    }
    int numDecodings(string s) {
        int n=s.size();
        if(n==0||s[0]=='0') return 0;
        int pre=1;
        int cur=1;
        int ret=0;
        for(int i=1;i<n;i++){
            ret=(check(s[i])?cur:0)+(check(s[i-1],s[i])?pre:0);
            pre=cur;
            cur=ret;
        }
        return cur;
    }
};