算法分析与设计课程——LeetCode刷题之Decode Ways

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题目：

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.

答案:

class Solution {
public:
     int n = s.size();
    if(n == 0 || s[0] == '0') return 0;
    if(n == 1) return 1;
    int res = 0,fn_1 = 1,fn_2 = 1;
    for(int i = 1;i < n;i++){
        int temp = fn_1;
        if(isValid(s[i])&&isValid(s[i-1],s[i]))  res+=fn_1+fn_2;
        if(!isValid(s[i])&&isValid(s[i-1],s[i])) res+=fn_2;
        if(isValid(s[i])&&!isValid(s[i-1],s[i])) res+=fn_1;
        if(!isValid(s[i])&&!isValid(s[i-1],s[i]))  return 0;
        fn_1 = res;
        fn_2 = temp;
        res = 0;
    }
    return fn_1;
}
bool isValid(char a,char b){
    return a == '1'||(a == '2' && b <='6');
}
bool isValid(char a){
    return a != '0';
}
};