LeetCode 题目：91. Decode Ways

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题目描述：

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.

代码：

bool isValid(char a,char b){    return a == '1'||(a == '2' && b <='6');}bool isValid(char a){    return a != '0';}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;   }};

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