[LeetCode]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.

Have you been asked this question in an interview? Yes

思考：未考虑0的情况wrong了好多次。

class Solution {
public:
    int numDecodings(string s) {
        //未考虑0的情况
        //"0","00","01","110"
        int len=s.size();
        if(len==0) return 0;
        if(s[0]=='0') return 0;
        vector<int> dp(len+1,0);
        if(len==1) return 1;
        dp[0]=1;int temp;
        if(s[1]=='0')
        {
            if(s[0]=='1'||s[0]=='2') dp[1]=1;
            else return 0;
        }
        else
        {
            temp=10*(s[0]-'0')+(s[1]-'0');
            if(temp>10&&temp<=26) dp[1]=2;
            else dp[1]=1;
        }
        for(int i=2;i<len;i++)
        {
            if(s[i]=='0')
            {
                if(s[i-1]=='0') return 0;
                else if(s[i-1]=='1'||s[i-1]=='2') dp[i]=dp[i-2];
                else return 0;
            }
            else
            {
                temp=10*(s[i-1]-'0')+(s[i]-'0');
                if(temp>10&&temp<=26) dp[i]=dp[i-1]+dp[i-2];
                else dp[i]=dp[i-1];
            }
        }
        return dp[len-1];
    }
};