CODE  Code
KEY Inc., the leading company in security hardware, has developed a new kind of safe. To unlock it, you don't need a key but you are required to enter the correct n
digit code on a keypad (as if this were something new!). There are several models available, from toy safes for children (with a 2digit code) to the military version (with a 6digit code).
The safe will open as soon as the last digit of the correct code is entered. There is no "enter" key. When you enter more than n
digits, only the last n
digits are significant. For example (in the 4digit version), if the correct code is 4567, and you plan to enter the digit sequence 1234567890, the door will open as soon as you press the 7 key.
The software to create this effect is rather simple. In the n
digit version the safe is always in one of 10^{n1}
internal states. The current state of the safe simply represents the last n1
digits that have been entered. One of these states (in the example above, state 456) is marked as the unlocked
state. If the safe is in the unlocked state and then the right key (in the example above, 7) is pressed, the door opens. Otherwise the safe shifts to the corresponding new state. For example, if the safe is in state 456 and then you press 8, the safe goes into state 568.
A trivial strategy to open the safe is to enter all possible codes one after the other. In the worst case, however, this will require n * 10^{n}
keystrokes. By choosing a good digit sequence it is possible to open the safe in at most 10^{n} + n  1
keystrokes. All you have to do is to find a digit sequence that contains all n
digit sequences exactly once. KEY Inc. claims that for the military version (n=6) the fastest computers available today would need billions of years to find such a sequence  but apparently they don't know what some programmers are capable of...
Input Specification
The input contains several test cases. Every test case is specified by an integer n
. You may assume that 1<=n<=6
. The last test case is followed by a zero.
Output Specification
For each test case specified by n
output a line containing a sequence of 10^{n} + n  1
digits that contains each n
digit sequence exactly once.
Sample Input
1 2 0
Sample Output
0123456789 00102030405060708091121314151617181922324252627282933435363738394454647484955657585966768697787988990
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xopxop1:
20170818 13:33:19
Solution to this and many other tasks are here http://xoptutorials.com/index.php/category/spoj/ 

krist7599555:
20170412 13:13:32
used stack instant of recursive for better time ;) 

blaz1ngra1n:
20150313 00:56:03
My correct solution using [...] algorithm is too slow (It takes 0.6 seconds to solve the N = 6 case. Very strict time limit.) Last edit: 20150314 18:44:31 
Added by:  Wanderley GuimarÄƒes 
Date:  20070919 
Time limit:  0.200s 
Source limit:  50000B 
Memory limit:  1536MB 
Cluster:  Cube (Intel G860) 
Languages:  All 
Resource:  University of Ulm Local Contest 2004 