[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.

思路：同爬楼梯问题，但是需要多一些判决条件

class Solution {public:    int numDecodings(string s) {        if (s.empty() || s[0] == '0') {            return 0;        }        int prev = 0;        int cur = 1;        for (int i = 1; i <= s.size(); i++) {            if (s[i-1] == '0') {                cur = 0;            }            if (i < 2 || !(s[i-2] == '1' || (s[i-2] == '2' && s[i-1] <= '6'))) {                prev = 0;            }            int tmp = cur;            cur += prev;            prev = tmp;        }        return cur;    }};