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.

动态规划

class Solution{public:int numDecodings(string s){if (s.find("00") != string::npos || s[0] == '0' || s.empty())return 0;if (s.size() == 1) return 1;int prev2 = 1;int prev1 = 0;if (atoi(s.substr(0, 2).c_str()) <= 26)prev1++;if (s[1] != '0')prev1++;for (int i = 2; i < s.length(); i++){int tmp = 0;    if (s[i] == '0'){if (s[i-1] > '2')return 0;}elsetmp = prev1;if (s[i - 1] != '0' && atoi(s.substr(i-1, 2).c_str()) <= 26)tmp += prev2;prev2 = prev1;prev1 = tmp;}return prev1;}};