Codeforces Round #189 (Div. 2) A. Magic Numbers【正难则反/给出一个数字串判断是否只由1,14和144组成】
A. Magic Numbers
time limit per test
2 seconds memory limit per test
256 megabytes input
standard input output
standard output 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 but1444, 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.
Examples
input
114114
output
YES
input
1111
output
YES
input
441231
output
NO
【代码】：
#include <bits/stdc++.h>
using namespace std;
string a;
int i,k;
bool f=true;
int main() {
cin >> a;
for (i=0;i<a.length();i++)
{
if(a[0]!='1') f=0; //首位非1开头 错
if(a[i]!='1'&&a[i]!='4') f=0;//有非1,4的字符出现 错
if(a[i]=='1') //开头为1 计数器k清零
k=0;
else if(a[i]=='4') //出现4个数超过2 错
{
k++;
if(k>2) f=0;
}
}
if (f) cout << "YES" << endl;
else cout << "NO" << endl;
return 0;
}
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