LeetCode —— Decode Ways

来源：互联网发布：mac office 2016 破解编辑：程序博客网时间：2024/06/01 08:45

链接：http://leetcode.com/onlinejudge#question_91

原题：

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：有点dp的思想，要小心不合法字符串到出现，我第一次就WA了。

考虑元素s[i]，只有两种可能：

1）单独解码，当然s[i] != '0'

2) 和s[i-1]结合了解码，要求 s[i-1]s[i]的值在10～26之间，不然不行。

PS：小心0，00的不合法字符串。

代码：

class Solution {public:    int numDecodings(string s) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if (s.size() == 0)            return 0;        if (s[0] == '0')            return 0;                vector<int> dp(s.size()+1, 0);        dp[0] = dp[1] = 1;                for (int n=2; n<=s.size(); n++) {            int i = n - 1;            if (s[i] != '0')                dp[n] = dp[n-1];            int code = (s[i-1]-'0')*10 + (s[i]-'0');            if (code == 0)                return 0;            else if (code>9 && code<27)                dp[n] += dp[n-2];        }                return dp[s.size()];    }};