題目

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.

頻度： 4

解題之法

class Solution {
public:
    int numDecodings(string s) {
        
      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';
}
};