leetcode No91. Decode Ways

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Question：

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.

有多少种解码可能，这里的'A'~'Z'已经被转化成数字字符了

Algorithm:

动态规划，但是要判断一些条件，具体见程序

Accepted Code:

class Solution {            public:    int numDecodings(string s) {        if(s.empty())            return 0;        int res[s.size()+1];        res[0]=1;        res[1]=(s[0]=='0'?0:1);        for(int i=2;i<=s.size();i++)        {            if(s[i-1]=='0')                res[i]=0;            else                res[i]=res[i-1];    //一个元素只有一个可能，所以继承前一位的            if(s[i-2]=='1'||(s[i-2]=='2'&&s[i-1]<='6'))   //1~26     //两种可能，可能性翻倍                res[i]=res[i]+res[i-2];        }        return res[s.size()];    }};

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