Add Knight's Tour (TheAlgorithms#2701)

Author: limbad-YK

Backtracking/KnightsTour.java

@@ -0,0 +1,126 @@
+package Backtracking;
+
+import java.util.*;
+
+/*
+    * Problem Statement: -
+    
+    Given a N*N board with the Knight placed on the first block of an empty board. Moving according to the rules of
+    chess knight must visit each square exactly once. Print the order of each cell in which they are visited.
+
+    Example: -
+
+    Input : N = 8
+
+    Output:
+        0  59  38  33  30  17   8  63
+        37  34  31  60   9  62  29  16
+        58   1  36  39  32  27  18   7
+        35  48  41  26  61  10  15  28
+        42  57   2  49  40  23   6  19
+        47  50  45  54  25  20  11  14
+        56  43  52   3  22  13  24   5
+        51  46  55  44  53   4  21  12
+
+ */
+
+public class KnightsTour {
+    private final static int base = 12;
+    private final static int[][] moves = {{1,-2},{2,-1},{2,1},{1,2},{-1,2},{-2,1},{-2,-1},{-1,-2}}; // Possible moves by knight on chess
+    private static int[][] grid;    // chess grid
+    private static int total;   // total squares in chess
+
+    public static void main(String[] args) {
+        grid = new int[base][base];
+        total = (base - 4) * (base - 4);
+
+        for (int r = 0; r < base; r++)
+            for (int c = 0; c < base; c++)
+                if (r < 2 || r > base - 3 || c < 2 || c > base - 3)
+                    grid[r][c] = -1;
+
+        int row = 2 + (int) (Math.random() * (base - 4));
+        int col = 2 + (int) (Math.random() * (base - 4));
+
+        grid[row][col] = 1;
+
+        if (solve(row, col, 2))
+            printResult();
+        else System.out.println("no result");
+
+    }
+
+    // Return True when solvable
+    private static boolean solve(int row, int column, int count) {
+        if (count > total)
+            return true;
+
+        List<int[]> neighbor = neighbors(row, column);
+
+        if (neighbor.isEmpty() && count != total)
+            return false;
+
+        Collections.sort(neighbor, new Comparator<int[]>() {
+            public int compare(int[] a, int[] b) {
+                return a[2] - b[2];
+            }
+        });
+
+        for (int[] nb : neighbor) {
+            row = nb[0];
+            column = nb[1];
+            grid[row][column] = count;
+            if (!orphanDetected(count, row, column) && solve(row, column, count + 1))
+                return true;
+            grid[row][column] = 0;
+        }
+
+        return false;
+    }
+
+    // Returns List of neighbours
+    private static List<int[]> neighbors(int row, int column) {
+        List<int[]> neighbour = new ArrayList<>();
+
+        for (int[] m : moves) {
+            int x = m[0];
+            int y = m[1];
+            if (grid[row + y][column + x] == 0) {
+                int num = countNeighbors(row + y, column + x);
+                neighbour.add(new int[]{row + y, column + x, num});
+            }
+        }
+        return neighbour;
+    }
+
+    // Returns the total count of neighbors
+    private static int countNeighbors(int row, int column) {
+        int num = 0;
+        for (int[] m : moves)
+            if (grid[row + m[1]][column + m[0]] == 0)
+                num++;
+        return num;
+    }
+
+    // Returns true if it is orphan
+    private static boolean orphanDetected(int count, int row, int column) {
+        if (count < total - 1) {
+            List<int[]> neighbor = neighbors(row, column);
+            for (int[] nb : neighbor)
+                if (countNeighbors(nb[0], nb[1]) == 0)
+                    return true;
+        }
+        return false;
+    }
+
+    // Prints the result grid
+    private static void printResult() {
+        for (int[] row : grid) {
+            for (int i : row) {
+                if (i == -1) continue;
+                System.out.printf("%2d ", i);
+            }
+            System.out.println();
+        }
+    }
+}