Given an NxN chessboard and a Knight at position (x,y). The Knight has to take exactly K steps, where at each step it chooses any of the 8 directions uniformly at random. What is the probability that the Knight remains in the chessboard after taking K steps, with the condition that it can’t enter the board again once it leaves it.
Let's take: 8x8 chessboard, initial position of the knight : (0, 0), number of steps : 1 At each step, the Knight has 8 different positions to choose from.
If it starts from (0, 0), after taking one step it will lie inside the board only at 2 out of 8 positions, and will lie outside at other positions. So, the probability is 2/8 = 0.25
One thing that we can observe is that at every step the Knight has 8 choices to choose from. Suppose, the Knight has to take k steps and after taking the Kth step the knight reaches (x,y). There are 8 different positions from where the Knight can reach to (x,y) in one step, and they are: (x+1,y+2), (x+2,y+1), (x+2,y-1), (x+1,y-2), (x-1,y-2), (x-2,y-1), (x-2,y+1), (x-1,y+2).
What if we already knew the probabilities of reaching these 8 positions after K-1 steps? Then, the final probability after K steps will simply be equal to the (Σ probability of reaching each of these 8 positions after K-1 steps)/8;
Here we are dividing by 8 because each of these 8 positions have 8 choices and position (x,y) is one of the choice.
For the positions that lie outside the board, we will either take their probabilities as 0 or simply neglect it.
Since, we need to keep track of the probabilities at each position for every number of steps, we need Dynamic Programming to solve this problem.
We are going to take an array dp[x][y][steps] which will store the probability of reaching (x,y) after (steps) number of moves.
Base case : if number of steps is 0, then the probability that the Knight will remain inside the board is 1.
Here is the implementation :
Time Complexity: O(NxNxKx8) which is O(NxNxK), where N is the size of the board and K is the number of steps.
Space Complexity: O(NxNxK)
This article is contributed by Avinash Kumar Saw. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to email@example.com. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
- Number of blocks in a chessboard a knight can move to in exactly k moves
- Possible moves of knight
- The Knight's tour problem | Backtracking-1
- Warnsdorff's algorithm for Knight’s tour problem
- Minimum steps to reach target by a Knight | Set 2
- Minimum steps to reach target by a Knight | Set 1
- Maximum bishops that can be placed on N*N chessboard
- Check if the given chessboard is valid or not
- Maximum non-attacking Knights that can be placed on an N*M Chessboard
- Check if a Queen can attack a given cell on chessboard
- Ways to place K bishops on an N×N chessboard so that no two attack
- Total position where king can reach on a chessboard in exactly M moves | Set 2
- Total position where king can reach on a chessboard in exactly M moves
- Probability of rain on N+1th day
- Probability of getting more value in third dice throw