leetcode 91. 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.

ret[i + 1] 代表到达第i位的路径数目

class Solution {public:    int numDecodings(string s)     {        if (s.size() == 0 || s[0] == '0')            return 0;                vector<int> ret(s.size() + 1, 1);        for (int i = 1; i < s.size(); i++)        {            if (s[i] == '0') //当前是0            {                if(s[i-1] != '1' && s[i-1] != '2')                    return 0;                else                    ret[i+1] = ret[i-1];                }            else if(s[i] >= '1' && s[i] <= '6')  //当前是1到6            {                if (s[i-1] == '1' || s[i-1] == '2')  //前面是1或者2，就有多路                    ret[i+1] = ret[i] + ret[i-1];                else                    ret[i+1] = ret[i];            }            else            {                if(s[i-1]=='1')                    ret[i+1] = ret[i] + ret[i-1];                else                    ret[i+1] = ret[i];            }        }        return ret[s.size()];     }};