בהינתן לוח שחמט מרובע בגודל N x N ניתנת למיקום של אביר ולמיקום של מטרה המשימה היא לגלות את הצעדים המינימליים שאביר ינקוט כדי להגיע למיקום המטרה.

דוגמאות:
Input : (2 4) - knight's position (6 4) - target cell Output : 2 Input : (4 5) (1 1) Output : 3
גישת BFS לפתרון הבעיה שלעיל כבר נדונה ב- קוֹדֵם שֶׁלְאַחַר. בפוסט זה נדון פתרון תכנות דינמי.
הסבר על הגישה:
תן לוח שחמט של 8 x 8 תאים. עכשיו נניח שהאביר נמצא ב-(3 3) והמטרה ב-(7 8). ישנם 8 מהלכים אפשריים מהמיקום הנוכחי של האביר, כלומר (2 1) (1 2) (4 1) (1 4) (5 2) (2 5) (5 4) (4 5). אבל בין אלה רק שני מהלכים (5 4) ו- (4 5) יהיו לעבר המטרה וכל השאר מתרחקים מהמטרה. אז למציאת שלבים מינימליים, עבור אל (4 5) או (5 4). כעת חשב את הצעדים המינימליים שנלקחו מ- (4 5) ו- (5 4) כדי להגיע ליעד. זה מחושב על ידי תכנות דינמי. לפיכך זה מביא לשלבים המינימליים מ- (3 3) ל- (7 8).
תן לוח שחמט של 8 x 8 תאים. עכשיו נניח שהאביר נמצא ב-(4 3) והמטרה ב-(4 7). יש 8 מהלכים אפשריים אבל לכיוון המטרה יש רק 4 מהלכים כלומר (5 5) (3 5) (2 4) (6 4). כמו (5 5) שווה ערך ל- (3 5) ו- (2 4) שווה ערך ל- (6 4). אז מ-4 הנקודות הללו ניתן להמיר אותו ל-2 נקודות. לוקח (5 5) ו- (6 4) (כאן). כעת חשב את הצעדים המינימליים שננקטו משתי הנקודות הללו כדי להגיע ליעד. זה מחושב על ידי תכנות דינמי. לפיכך זה מביא לשלבים המינימליים מ- (4 3) ל- (4 7).
חריג: כאשר האביר יהיה בפינה והמטרה היא כזו שההפרש של קואורדינטות x ו-y עם מיקום האביר הוא (1 1) או להיפך. אז הצעדים המינימליים יהיו 4.
משוואת תכנות דינמית:
1) dp[diffOfX][diffOfY] הוא הצעדים המינימליים שנלקחו ממיקום האביר למיקום המטרה.
2) dp[diffOfX][diffOfY] = dp[diffOfY][diffOfX] .
כאשר diffOfX = הבדל בין קואורדינטת ה-x של האביר לקואורדינטת ה-X של המטרה
diffOfY = הבדל בין קואורדינטת ה-y של האביר לקואורדינטת ה-y של המטרה
להלן יישום הגישה לעיל:
// C++ code for minimum steps for // a knight to reach target position #include using namespace std; // initializing the matrix. int dp[8][8] = { 0 }; int getsteps(int x int y int tx int ty) { // if knight is on the target // position return 0. if (x == tx && y == ty) return dp[0][0]; else { // if already calculated then return // that value. Taking absolute difference. if (dp[abs(x - tx)][abs(y - ty)] != 0) return dp[abs(x - tx)][abs(y - ty)]; else { // there will be two distinct positions // from the knight towards a target. // if the target is in same row or column // as of knight then there can be four // positions towards the target but in that // two would be the same and the other two // would be the same. int x1 y1 x2 y2; // (x1 y1) and (x2 y2) are two positions. // these can be different according to situation. // From position of knight the chess board can be // divided into four blocks i.e.. N-E E-S S-W W-N . if (x <= tx) { if (y <= ty) { x1 = x + 2; y1 = y + 1; x2 = x + 1; y2 = y + 2; } else { x1 = x + 2; y1 = y - 1; x2 = x + 1; y2 = y - 2; } } else { if (y <= ty) { x1 = x - 2; y1 = y + 1; x2 = x - 1; y2 = y + 2; } else { x1 = x - 2; y1 = y - 1; x2 = x - 1; y2 = y - 2; } } // ans will be 1 + minimum of steps // required from (x1 y1) and (x2 y2). dp[abs(x - tx)][abs(y - ty)] = min(getsteps(x1 y1 tx ty) getsteps(x2 y2 tx ty)) + 1; // exchanging the coordinates x with y of both // knight and target will result in same ans. dp[abs(y - ty)][abs(x - tx)] = dp[abs(x - tx)][abs(y - ty)]; return dp[abs(x - tx)][abs(y - ty)]; } } } // Driver Code int main() { int i n x y tx ty ans; // size of chess board n*n n = 100; // (x y) coordinate of the knight. // (tx ty) coordinate of the target position. x = 4; y = 5; tx = 1; ty = 1; // (Exception) these are the four corner points // for which the minimum steps is 4. if ((x == 1 && y == 1 && tx == 2 && ty == 2) || (x == 2 && y == 2 && tx == 1 && ty == 1)) ans = 4; else if ((x == 1 && y == n && tx == 2 && ty == n - 1) || (x == 2 && y == n - 1 && tx == 1 && ty == n)) ans = 4; else if ((x == n && y == 1 && tx == n - 1 && ty == 2) || (x == n - 1 && y == 2 && tx == n && ty == 1)) ans = 4; else if ((x == n && y == n && tx == n - 1 && ty == n - 1) || (x == n - 1 && y == n - 1 && tx == n && ty == n)) ans = 4; else { // dp[a][b] here a b is the difference of // x & tx and y & ty respectively. dp[1][0] = 3; dp[0][1] = 3; dp[1][1] = 2; dp[2][0] = 2; dp[0][2] = 2; dp[2][1] = 1; dp[1][2] = 1; ans = getsteps(x y tx ty); } cout << ans << endl; return 0; }
Java //Java code for minimum steps for // a knight to reach target position public class GFG { // initializing the matrix. static int dp[][] = new int[8][8]; static int getsteps(int x int y int tx int ty) { // if knight is on the target // position return 0. if (x == tx && y == ty) { return dp[0][0]; } else // if already calculated then return // that value. Taking absolute difference. if (dp[ Math.abs(x - tx)][ Math.abs(y - ty)] != 0) { return dp[ Math.abs(x - tx)][ Math.abs(y - ty)]; } else { // there will be two distinct positions // from the knight towards a target. // if the target is in same row or column // as of knight then there can be four // positions towards the target but in that // two would be the same and the other two // would be the same. int x1 y1 x2 y2; // (x1 y1) and (x2 y2) are two positions. // these can be different according to situation. // From position of knight the chess board can be // divided into four blocks i.e.. N-E E-S S-W W-N . if (x <= tx) { if (y <= ty) { x1 = x + 2; y1 = y + 1; x2 = x + 1; y2 = y + 2; } else { x1 = x + 2; y1 = y - 1; x2 = x + 1; y2 = y - 2; } } else if (y <= ty) { x1 = x - 2; y1 = y + 1; x2 = x - 1; y2 = y + 2; } else { x1 = x - 2; y1 = y - 1; x2 = x - 1; y2 = y - 2; } // ans will be 1 + minimum of steps // required from (x1 y1) and (x2 y2). dp[ Math.abs(x - tx)][ Math.abs(y - ty)] = Math.min(getsteps(x1 y1 tx ty) getsteps(x2 y2 tx ty)) + 1; // exchanging the coordinates x with y of both // knight and target will result in same ans. dp[ Math.abs(y - ty)][ Math.abs(x - tx)] = dp[ Math.abs(x - tx)][ Math.abs(y - ty)]; return dp[ Math.abs(x - tx)][ Math.abs(y - ty)]; } } // Driver Code static public void main(String[] args) { int i n x y tx ty ans; // size of chess board n*n n = 100; // (x y) coordinate of the knight. // (tx ty) coordinate of the target position. x = 4; y = 5; tx = 1; ty = 1; // (Exception) these are the four corner points // for which the minimum steps is 4. if ((x == 1 && y == 1 && tx == 2 && ty == 2) || (x == 2 && y == 2 && tx == 1 && ty == 1)) { ans = 4; } else if ((x == 1 && y == n && tx == 2 && ty == n - 1) || (x == 2 && y == n - 1 && tx == 1 && ty == n)) { ans = 4; } else if ((x == n && y == 1 && tx == n - 1 && ty == 2) || (x == n - 1 && y == 2 && tx == n && ty == 1)) { ans = 4; } else if ((x == n && y == n && tx == n - 1 && ty == n - 1) || (x == n - 1 && y == n - 1 && tx == n && ty == n)) { ans = 4; } else { // dp[a][b] here a b is the difference of // x & tx and y & ty respectively. dp[1][0] = 3; dp[0][1] = 3; dp[1][1] = 2; dp[2][0] = 2; dp[0][2] = 2; dp[2][1] = 1; dp[1][2] = 1; ans = getsteps(x y tx ty); } System.out.println(ans); } } /*This code is contributed by PrinciRaj1992*/
Python3 # Python3 code for minimum steps for # a knight to reach target position # initializing the matrix. dp = [[0 for i in range(8)] for j in range(8)]; def getsteps(x y tx ty): # if knight is on the target # position return 0. if (x == tx and y == ty): return dp[0][0]; # if already calculated then return # that value. Taking absolute difference. elif(dp[abs(x - tx)][abs(y - ty)] != 0): return dp[abs(x - tx)][abs(y - ty)]; else: # there will be two distinct positions # from the knight towards a target. # if the target is in same row or column # as of knight then there can be four # positions towards the target but in that # two would be the same and the other two # would be the same. x1 y1 x2 y2 = 0 0 0 0; # (x1 y1) and (x2 y2) are two positions. # these can be different according to situation. # From position of knight the chess board can be # divided into four blocks i.e.. N-E E-S S-W W-N . if (x <= tx): if (y <= ty): x1 = x + 2; y1 = y + 1; x2 = x + 1; y2 = y + 2; else: x1 = x + 2; y1 = y - 1; x2 = x + 1; y2 = y - 2; elif (y <= ty): x1 = x - 2; y1 = y + 1; x2 = x - 1; y2 = y + 2; else: x1 = x - 2; y1 = y - 1; x2 = x - 1; y2 = y - 2; # ans will be 1 + minimum of steps # required from (x1 y1) and (x2 y2). dp[abs(x - tx)][abs(y - ty)] = min(getsteps(x1 y1 tx ty) getsteps(x2 y2 tx ty)) + 1; # exchanging the coordinates x with y of both # knight and target will result in same ans. dp[abs(y - ty)][abs(x - tx)] = dp[abs(x - tx)][abs(y - ty)]; return dp[abs(x - tx)][abs(y - ty)]; # Driver Code if __name__ == '__main__': # size of chess board n*n n = 100; # (x y) coordinate of the knight. # (tx ty) coordinate of the target position. x = 4; y = 5; tx = 1; ty = 1; # (Exception) these are the four corner points # for which the minimum steps is 4. if ((x == 1 and y == 1 and tx == 2 and ty == 2) or (x == 2 and y == 2 and tx == 1 and ty == 1)): ans = 4; elif ((x == 1 and y == n and tx == 2 and ty == n - 1) or (x == 2 and y == n - 1 and tx == 1 and ty == n)): ans = 4; elif ((x == n and y == 1 and tx == n - 1 and ty == 2) or (x == n - 1 and y == 2 and tx == n and ty == 1)): ans = 4; elif ((x == n and y == n and tx == n - 1 and ty == n - 1) or (x == n - 1 and y == n - 1 and tx == n and ty == n)): ans = 4; else: # dp[a][b] here a b is the difference of # x & tx and y & ty respectively. dp[1][0] = 3; dp[0][1] = 3; dp[1][1] = 2; dp[2][0] = 2; dp[0][2] = 2; dp[2][1] = 1; dp[1][2] = 1; ans = getsteps(x y tx ty); print(ans); # This code is contributed by PrinciRaj1992
C# // C# code for minimum steps for // a knight to reach target position using System; public class GFG{ // initializing the matrix. static int [ ]dp = new int[8 8]; static int getsteps(int x int y int tx int ty) { // if knight is on the target // position return 0. if (x == tx && y == ty) { return dp[0 0]; } else // if already calculated then return // that value. Taking Absolute difference. if (dp[ Math. Abs(x - tx) Math. Abs(y - ty)] != 0) { return dp[ Math. Abs(x - tx) Math. Abs(y - ty)]; } else { // there will be two distinct positions // from the knight towards a target. // if the target is in same row or column // as of knight then there can be four // positions towards the target but in that // two would be the same and the other two // would be the same. int x1 y1 x2 y2; // (x1 y1) and (x2 y2) are two positions. // these can be different according to situation. // From position of knight the chess board can be // divided into four blocks i.e.. N-E E-S S-W W-N . if (x <= tx) { if (y <= ty) { x1 = x + 2; y1 = y + 1; x2 = x + 1; y2 = y + 2; } else { x1 = x + 2; y1 = y - 1; x2 = x + 1; y2 = y - 2; } } else if (y <= ty) { x1 = x - 2; y1 = y + 1; x2 = x - 1; y2 = y + 2; } else { x1 = x - 2; y1 = y - 1; x2 = x - 1; y2 = y - 2; } // ans will be 1 + minimum of steps // required from (x1 y1) and (x2 y2). dp[ Math. Abs(x - tx) Math. Abs(y - ty)] = Math.Min(getsteps(x1 y1 tx ty) getsteps(x2 y2 tx ty)) + 1; // exchanging the coordinates x with y of both // knight and target will result in same ans. dp[ Math. Abs(y - ty) Math. Abs(x - tx)] = dp[ Math. Abs(x - tx) Math. Abs(y - ty)]; return dp[ Math. Abs(x - tx) Math. Abs(y - ty)]; } } // Driver Code static public void Main() { int i n x y tx ty ans; // size of chess board n*n n = 100; // (x y) coordinate of the knight. // (tx ty) coordinate of the target position. x = 4; y = 5; tx = 1; ty = 1; // (Exception) these are the four corner points // for which the minimum steps is 4. if ((x == 1 && y == 1 && tx == 2 && ty == 2) || (x == 2 && y == 2 && tx == 1 && ty == 1)) { ans = 4; } else if ((x == 1 && y == n && tx == 2 && ty == n - 1) || (x == 2 && y == n - 1 && tx == 1 && ty == n)) { ans = 4; } else if ((x == n && y == 1 && tx == n - 1 && ty == 2) || (x == n - 1 && y == 2 && tx == n && ty == 1)) { ans = 4; } else if ((x == n && y == n && tx == n - 1 && ty == n - 1) || (x == n - 1 && y == n - 1 && tx == n && ty == n)) { ans = 4; } else { // dp[a b] here a b is the difference of // x & tx and y & ty respectively. dp[1 0] = 3; dp[0 1] = 3; dp[1 1] = 2; dp[2 0] = 2; dp[0 2] = 2; dp[2 1] = 1; dp[1 2] = 1; ans = getsteps(x y tx ty); } Console.WriteLine(ans); } } /*This code is contributed by PrinciRaj1992*/
JavaScript <script> // JavaScript code for minimum steps for // a knight to reach target position // initializing the matrix. let dp = new Array(8) for(let i=0;i<8;i++){ dp[i] = new Array(8).fill(0) } function getsteps(xytxty) { // if knight is on the target // position return 0. if (x == tx && y == ty) return dp[0][0]; else { // if already calculated then return // that value. Taking absolute difference. if (dp[(Math.abs(x - tx))][(Math.abs(y - ty))] != 0) return dp[(Math.abs(x - tx))][(Math.abs(y - ty))]; else { // there will be two distinct positions // from the knight towards a target. // if the target is in same row or column // as of knight then there can be four // positions towards the target but in that // two would be the same and the other two // would be the same. let x1 y1 x2 y2; // (x1 y1) and (x2 y2) are two positions. // these can be different according to situation. // From position of knight the chess board can be // divided into four blocks i.e.. N-E E-S S-W W-N . if (x <= tx) { if (y <= ty) { x1 = x + 2; y1 = y + 1; x2 = x + 1; y2 = y + 2; } else { x1 = x + 2; y1 = y - 1; x2 = x + 1; y2 = y - 2; } } else { if (y <= ty) { x1 = x - 2; y1 = y + 1; x2 = x - 1; y2 = y + 2; } else { x1 = x - 2; y1 = y - 1; x2 = x - 1; y2 = y - 2; } } // ans will be 1 + minimum of steps // required from (x1 y1) and (x2 y2). dp[(Math.abs(x - tx))][(Math.abs(y - ty))] = Math.min(getsteps(x1 y1 tx ty) getsteps(x2 y2 tx ty)) + 1; // exchanging the coordinates x with y of both // knight and target will result in same ans. dp[(Math.abs(y - ty))][(Math.abs(x - tx))] = dp[(Math.abs(x - tx))][(Math.abs(y - ty))]; return dp[(Math.abs(x - tx))][(Math.abs(y - ty))]; } } } // Driver Code let i n x y tx ty ans; // size of chess board n*n n = 100; // (x y) coordinate of the knight. // (tx ty) coordinate of the target position. x = 4; y = 5; tx = 1; ty = 1; // (Exception) these are the four corner points // for which the minimum steps is 4. if ((x == 1 && y == 1 && tx == 2 && ty == 2) || (x == 2 && y == 2 && tx == 1 && ty == 1)) ans = 4; else if ((x == 1 && y == n && tx == 2 && ty == n - 1) || (x == 2 && y == n - 1 && tx == 1 && ty == n)) ans = 4; else if ((x == n && y == 1 && tx == n - 1 && ty == 2) || (x == n - 1 && y == 2 && tx == n && ty == 1)) ans = 4; else if ((x == n && y == n && tx == n - 1 && ty == n - 1) || (x == n - 1 && y == n - 1 && tx == n && ty == n)) ans = 4; else { // dp[a][b] here a b is the difference of // x & tx and y & ty respectively. dp[1][0] = 3; dp[0][1] = 3; dp[1][1] = 2; dp[2][0] = 2; dp[0][2] = 2; dp[2][1] = 1; dp[1][2] = 1; ans = getsteps(x y tx ty); } document.write(ans''); // This code is contributed by shinjanpatra. </script>
תְפוּקָה:
3
מורכבות זמן: O(N * M) כאשר N הוא המספר הכולל של השורות ו-M הוא המספר הכולל של העמודות
מרחב עזר: O(N * M)