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Bulls and Cows Game

This game I learned during my junior college days and used to play often in crowded local trains on our way to and fro college.  Very little is known about this game.  This game comes handy in boring MACR classes.  :D

The game is played between two players. The rules of the games are like this:

One person thinks about a four digit number such that

The number to be filled in each position can be any number from 0-9
No two digits are repeated
The number to be a four digit number, in other words first digit cannot be zero

You then make an intelligent guess of that number.
After every guess, first person will tell the number of Bulls and cows. Number of cows signifies that as many digit in the guessed number /are right but are not in correct positions. Number of bulls signifies that number of digits in the guessed number is/are right and are also at correct positions. However a correct number at right position will be considered a bull and not a cow.
The game continues with second guess and so on until the correct number is reached.
The objective of the game is to guess the correct number in minimum possible guesses.

Illustration:
Suppose the number is 1947:
Guess 1:  1573      1 Bull & 1 Cow (1 is correct digit at correct place and  7 is correct number but wrong place)
Guess 2:  2479        3 Cows  (4,7and 9 are correct numbers but  at wrong places)
Guess 3: 1487          2 Bull 1 Cow
Guess 4 : 1947        Correct Answer!!!!!!!!!!!!!!!!!
Read more about this game : http://en.wikipedia.org/wiki/Bulls_and_cows