25. A $1 \times n$ chessboard is to be colored using red, white, and blue, with no two adjacent squares being red. Find the number of coloring schemes.
25. Let the number of coloring schemes be $f_{n}$. If the first cell is painted red, then the second cell can only be painted white or blue, and the remaining $1 \times(n-2)$ has $f_{n-2}$ coloring schemes. If the first cell is painted white or blue, then the remaining $1 \times(n-1)$ has $f_{n-1}$ coloring schemes. Therefore, $f_{n}=2 f_{n-1}+2 f_{n-2}$.