← Complete problem index

PROJECT EULER · #0306

Paper-strip Game

Statement only · SolvedOriginal problem ↗

The following game is a classic example of Combinatorial Game Theory:

Two players start with a strip of n white squares and they take alternate turns.
On each turn, a player picks two contiguous white squares and paints them black.
The first player who cannot make a move loses.

  • n=1: No valid moves, so the first player loses automatically.
  • n=2: Only one valid move, after which the second player loses.
  • n=3: Two valid moves, but both leave a situation where the second player loses.
  • n=4: Three valid moves for the first player, who is able to win the game by painting the two middle squares.
  • n=5: Four valid moves for the first player (shown below in red), but no matter what the player does, the second player (blue) wins.
0306_pstrip.gif

So, for 1n5, there are 3 values of n for which the first player can force a win.
Similarly, for 1n50, there are 40 values of n for which the first player can force a win.

For 1n1000000, how many values of n are there for which the first player can force a win?

Write-up coming later

The complete problem is available here. An approach, code, and answer will be added later.