Reverse of a String

#include<stdio.h>

void main()

{

    char a[20];

    char b[20];

    int i=-1,j=0;// i =-1 because index starts from zero.

    printf("enter the string :- ");

   gets(a);  

    while(a[++i]!='\0'); // i denotes the last character of the string through this loop

    while(i>=0)

     b[j++] = a[--i];//copy last character of "a" char array string as the first character into the "b" char array. 

    b[j]='\0'; //insert null to terminate the string

    printf("Reverse  string is : %s",b);

}