Leetcode140: Decode Ways

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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.

动态规划的思路：

如果当前位置n的字符串若不为0，那么包含前n-1个字符串组成的解码个数；

若n-1位的字符串不为0，且n-1和n位可组成小于等于26的2位数，包含前n-2个字符串组成的解码个数。

class Solution {
public:
    int numDecodings(string s) {
        int n = s.size();
        if(n==0)    return 0;
        else if(n == 1)
            return (s[0]!='0')?1:0;
        vector<int> f(n);
        f[0] = (s[0]!='0')?1:0;
        f[1] = (s[1]!='0' && s[0]!='0'?1:0) + ((s[0]-'0')*10+(s[1]-'0')<=26 && (s[0]!='0')? 1:0);
        for(int i = 2; i < n; i++)
        {
            f[i] = ((s[i]!='0')?f[i-1]:0) + ((s[i-1]-'0')*10+(s[i]-'0')<=26 && (s[i-1]!='0')? f[i-2]:0);
        }
        return f[n-1];
    }
};