[LeetCode]91.Decode Ways

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.

分析

代码

/*---------------------------------------
*   日期：2015-06-23
*   作者：SJF0115
*   题目: 91.Decode Ways
*   网址：https://leetcode.com/problems/decode-ways/
*   结果：AC
*   来源：LeetCode
*   博客：
-----------------------------------------*/
#include 
   
     using namespace std; class Solution { public: int numDecodings(string s) { int size = s.size(); if(s[0] == '0'){ return 0; }//if if(size == 0 || size == 1){ return size; }//if int pre = 1,cur = 1,res; for(int i = 1;i < size;++i){ if(isValid(s[i-1],s[i]) && isValid(s[i])){ res = pre + cur; }//if else if(!isValid(s[i-1],s[i]) && isValid(s[i])){ res = cur; }//else else if(isValid(s[i-1],s[i]) && !isValid(s[i])){ res = pre; }//else else{ return 0; }//else pre = cur; cur = res; }//for return res; } private: bool isValid(char pre,char cur){ if(pre == '1' || (pre == '2' && cur <= '6')){ return true; }//if return false; } bool isValid(char cur){ if(cur >= '1' && cur <= '9'){ return true; }//if return false; } }; int main(){ Solution s; string str("1202111110"); cout<
    
   

运行时间

思路2–超时

/*--------------------------------------- * 日期：2015-06-21 * 作者：SJF0115 * 题目: 91.Decode Ways * 网址：https://leetcode.com/problems/decode-ways/ * 结果：超时 * 来源：LeetCode * 博客： -----------------------------------------*/ #include 
    
      using namespace std; class Solution { public: int numDecodings(string s) { int size = s.size(); if(size == 0 || size == 1){ return size; }//if int index = -1; int count = 0; helper(s,index,count,""); return count; } private: void helper(string &s,int index,int &count,string word){ int size = s.size(); if(index == size-1){ ++count; cout<<"word->"<
     
       0){ word += ('A'+num-1); helper(s,index+i,count,word); word.erase(word.size()-1); }//if else{ break; } }//if }//for } }; int main(){ Solution s; string str("1234"); cout<