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  • [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

    这里写图片描写叙述

    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;
            }//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<<s.numDecodings(str)<<endl;
        return 0;
    }
    

    执行时间

    这里写图片描写叙述

    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;
            }//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->"<<word<<endl;
                return;
            }//if
            // 步长为1或者2
            int 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);
                    }//if
                    else{
                        break;
                    }
                }//if
            }//for
        }
    };
    
    int main(){
        Solution s;
        string str("1234");
        cout<<s.numDecodings(str)<<endl;
        return 0;
    }
    
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  • 原文地址:https://www.cnblogs.com/liguangsunls/p/7206445.html
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