Reverse string

/* C programme to print reversely a given string */
#include<stdio.h>
#include<string.h>
int main(void)
{
    int i,j=0,x;
    char a[999999];
    char b[999999];
    printf("Please enter string:\n");
    gets(a);
    x=strlen(a)-1;
    for(i=x;i>=0;i--)
    {
        b[j]=a[i];
        j++;
    }
    b[j]='\0';
    puts(b);
    return 0;
}