Decode Ways

题目描述为：

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.

简单DP：

int numDecodings(string s){if (s.empty())return 0;int n=s.size();vector<int> dp(n+1,0);dp[n]=1;int i=n-1;dp[i]=s[i]=='0'?0:1;for(i=n-2;i>=0;i--){if ( s[i]=='0' ){dp[i]=0;continue;}int tmp1=dp[i+1];int c=(s[i]-'0')*10+s[i+1]-'0';int tmp2=0;if ( c<=26 )tmp2=dp[i+2];dp[i]=tmp1+tmp2;}return dp[0];}