Codeforces Round #189 (Div. 2)---A. Magic Numbers_a magic number is a number formed by concatenation-优快云博客

本文链接：https://blog.youkuaiyun.com/u013446688/article/details/40607251

本文介绍了一种判断特定形式的魔法数字的方法。魔法数字是由1、14和144这三个数字组合而成，通过贪心算法来判断给定数字是否符合魔法数字的定义。文章提供了完整的AC代码实现。

A magic number is a number formed by concatenation of numbers 1, 14 and 144. We can use each of these numbers any number of times. Therefore 14144, 141414 and 1411 are magic numbers but 1444, 514 and 414 are not.

You're given a number. Determine if it is a magic number or not.

Input

The first line of input contains an integer n, (1 ≤ n ≤ 109). This number doesn't contain leading zeros.

Output

Print "YES" if n is a magic number or print "NO" if it's not.

   Sample test(s)
 
     input
   
114114

     output
   
YES

     input
   
1111

     output
   
YES

     input
   
441231

     output
   
NO

解题思路：贪心。每次都考虑对自己最优的情况，每种情况全考虑一下。

AC代码：

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <string>
#include <math.h>
#include <stdlib.h>
#include <time.h>
using namespace std;
#define INF 0x7fffffff

int main()
{
    #ifdef sxk
        freopen("in.txt","r",stdin);
    #endif
    string s;
    while(cin>>s)
    {
        int flag = 1;
        int len = s.size();
        for(int i=0; i<len; i++){
            if(s[i]=='4'){
                if((i>=1 && s[i-1]=='1') || (i>1 && s[i-2]=='1'))
                    continue;
                else{
                    flag = 0;
                    break;
                }
            }
            else if(s[i]!='1'){
                flag = 0;
                break;
            }
        }
        if(!flag)  cout<<"NO"<<endl;
        else  cout<<"YES"<<endl;
    }
    return 0;
}