A. Magic Numbers
time limit per test
2 secondsmemory limit per test
256 megabytesinput
standard inputoutput
standard outputA 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;
}