Leetcode: 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.

应该是DP，找dp[i]与dp[i-1]和dp[i-2]的关系。

class Solution {public:    int numDecodings(string s) {        if (s.empty() || s[0] == '0') {            return 0;        }                int prev1 = 1, prev2 = 1, cur = 0;        for (int i = 1; i < s.size(); ++i) {            if (s[i] == '0') {                if(s[i-1] == '1' || s[i-1] == '2') {                    cur = prev2;                }                else {                    return 0;                }            }            else {                cur = prev1;                if (s[i-1] == '1' || (s[i-1] == '2' && s[i] < '7')) {                    cur += prev2;                }            }            prev2 = prev1;            prev1 = cur;        }                return prev1;    }};