[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 <iostream>
using namespace std;class Solution {
public:int numDecodings(string s) {int size = s.size();if(s[0] == '0'){return 0;}//ifif(size == 0 || size == 1){return size;}//ifint 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;}//ifelse if(!isValid(s[i-1],s[i]) && isValid(s[i])){res = cur;}//elseelse if(isValid(s[i-1],s[i]) && !isValid(s[i])){res = pre;}//elseelse{return 0;}//elsepre = cur;cur = res;}//forreturn res;}
private:bool isValid(char pre,char cur){if(pre == '1' || (pre == '2' && cur <= '6')){return true;}//ifreturn false;}bool isValid(char cur){if(cur >= '1' && cur <= '9'){return true;}//ifreturn false;}
};int main(){Solution s;string str("1202111110");cout<<s.numDecodings(str)<<endl;return 0;
}

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 <iostream>
using namespace std;class Solution {public:int numDecodings(string s) {int size = s.size();if(size == 0 || size == 1){return size;}//ifint 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->"<<word<<endl;return;}//if// 步长为1或者2int num = 0;for(int i = 1;i <= 2;++i){if(index + i < size){num = num * 10 + s[index+i] - '0';if(num <= 26 && num > 0){word += ('A'+num-1);helper(s,index+i,count,word);word.erase(word.size()-1);}//ifelse{break;}}//if}//for}
};int main(){Solution s;string str("1234");cout<<s.numDecodings(str)<<endl;return 0;
}

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