leetcode91 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.

class Solution {//只考虑需要提取的情况，不要混乱
public:
int numDecodings(string s) {
int ans=0;
int len=s.length();
if(!len||s[0]=='0')
return 0;
vector<int> j(len,0);
j[0]=1;
for(int i=1;i<len;i++)
{
if(s[i]>='1'&&s[i]<='9')//单独解码的情况
j[i]+=j[i-1];
if(s[i]==0&&(s[i-1]==0||s[i-1]>='3'&&s[i-1]<='9'))
return 0;
int t=(s[i-1]-'0')*10+s[i]-'0';
if(t>=10&&t<=26) //合并解码的情况
{
if(i==1)
j[i]++;
else
j[i]+=j[i-2];
}

}
return j[len-1];
}
};

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