oj上一道题，自己测试没问题，oj结果是Segment Fault

源码如下，求大神解答，感激不尽
#include<stdio.h>
#include<stdlib.h>
int main()
{
long n,i,result=0,*a=(long *)malloc(1000000*sizeof(long)),*b=(long *)malloc(1000000*sizeof(long));
while(scanf("%ld",&n)!=EOF)
{
for(i=0;i<1000000;i++)
b[i]=0;
result=0;
for(i=0;i<n;i++)
{
scanf("%ld",&a[i]);
b[a[i]]++;
if(b[a[i]]>=n/2)
result=a[i];
}
printf("%ld\n",result);
}
//system("pause");
}