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.
字符A-Z用整数1-26编码，先给定一个数字串，问共有多少种解码情况
class Solution {
public:
bool isValid(string num){
if(num.length()==1 && num>="1" && num<="9")return true;
if(num.length()==2 && num>="10" && num<="26")return true;
return false;
}
int numDecodings(string s) {
if(s.length()==0)return 0;
if(s.length()==1){
if(isValid(s))return 1;
else return 0;//如果第1位为0，不可能解码
}
int *decodeWays = new int[s.length()];
//初始化解码到第1位的方案数
if(isValid(s.substr(0,1)))decodeWays[0]=1;
else return 0;
//初始化解码到第2位的方案数
decodeWays[1] = isValid(s.substr(1,1))?decodeWays[0]:0;
decodeWays[1] += isValid(s.substr(0,2))?1:0;
if(decodeWays[1]==0)return 0; //不存在解码到这一位的方案
//计算解码到第3~第s.length()位的方案数
for(int i=2; i<s.length(); i++){
decodeWays[i]=isValid(s.substr(i,1))?decodeWays[i-1]:0;
decodeWays[i]+=isValid(s.substr(i-1,2))?decodeWays[i-2]:0;
if(decodeWays[i]==0)return 0; //不存在解码到这一位的方案
}
return decodeWays[s.length()-1];
}
};LeetCode: Decode Ways [090],布布扣,bubuko.com
原文：http://blog.csdn.net/harryhuang1990/article/details/27817961