Added a check so that food will not appear on snake body.

[purnatadikonda5]

Solves issue #9 This code will work with O(1) complexity. Every time when the snake eats food then it will generate coordinates for food and it will check if at that particular coordinates if the snake exists.

CHANGES MADE: Created a function named "check" which will take the array as input and this array is [18][18] which will store 1 if snake exists at that particular coordinates else 0. When snake eats food it will call check function and this will generate a random coordinates and checks if there arr[x][y]=1 therefore working with an O(1) complexity.
If it is 1 then it will again call check function until arr[x][y] is 0.

[ANKITy102]

@purnatadikonda5, nice work!