decode-ways

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题目描述

A message containing letters fromA-Zis 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.

注意不合法的情况比如00,30,这种,利用dp

class Solution {public:    int numDecodings(string s) {        if(s.size()<1)            return 0;        if(s[0]=='0')            return 0;        vector<int> dp(s.size(),0);        dp[0]=1;        int tmp=atoi(s.substr(0,2).c_str());        if(tmp<=26&&tmp>0&&s[1]!='0')            dp[1]=2;        else if(s[1]=='0'&&s[0]>'2')            return 0;        else             dp[1]=1;        for(int i=2;i<s.size();++i)            {            int num=atoi(s.substr(i-1,2).c_str());            if(num<=26&&num>10&&s[i]!='0')                dp[i]=dp[i-1]+dp[i-2];            else if(s[i]=='0'&&s[i-1]!='0'&&num<=26)                dp[i]=dp[i-2];            else if(s[i]!='0')                dp[i]=dp[i-1];            else            return 0;        }        return dp[s.size()-1];            }};

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