solves #52: n queens ii in java

Author: anishLearnsToCode

README.md

@@ -58,6 +58,7 @@
 |  49  | [Group Anagrams](https://leetcode.com/problems/group-anagrams)                                                                                                         |                                                                                        [![Java](assets/java.png)](src/GroupAnagrams.java)                                                                                        |                                                                                                                                   |
 |  50  | [Pow(x,n)](https://leetcode.com/problems/powx-n)                                                                                                                       |                                                                                            [![Java](assets/java.png)](src/Powxn.java)                                                                                            |                                                                                                                                   |
 |  51  | [N-Queens](https://leetcode.com/problems/n-queens)                                                                                                                     |                                                                                           [![Java](assets/java.png)](src/NQueens.java)                                                                                           |                                                                                                                                   |
+|  51  | [N-Queens II](https://leetcode.com/problems/n-queens-ii)                                                                                                               |                                                                                          [![Java](assets/java.png)](src/NQueensII.java)                                                                                          |                                                                                                                                   |
 |  53  | [Maximum SubArray](https://leetcode.com/problems/maximum-subarray)                                                                                                     |                                                       [![Java](assets/java.png)](src/MaximumSubArray.java) [![Python](assets/python.png)](python/maximum_sum_subarray.py)                                                        |                                                                                                                                   |
 |  54  | [Spiral Matrix](https://leetcode.com/problems/spiral-matrix)                                                                                                           |                                                                                        [![Java](assets/java.png)](src/SpiralMatrix.java)                                                                                         |                                                                                                                                   |
 |  55  | [Jump Game](https://leetcode.com/problems/jump-game)                                                                                                                   |                                                                                          [![Java](assets/java.png)](src/JumpGame.java)                                                                                           |                                                                                                                                   |

src/NQueensII.java

@@ -0,0 +1,97 @@
+// https://leetcode.com/problems/n-queens-ii
+// T: O(N!)
+// S: O(N^2)
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class NQueensII {
+    private static boolean[] rows, columns;
+    private static int result = 0;
+
+    public int totalNQueens(int n) {
+        result = 0;
+        final List<String> board = getEmptyBoard(n);
+        rows = new boolean[n];
+        columns = new boolean[n];
+        nQueens(0, n, board, 0);
+        return result;
+    }
+
+    private static void nQueens(int row, int n, List<String> board, int queens) {
+        if (row == n) {
+            if (queens == n) {
+                result++;
+            }
+            return;
+        }
+
+        for (int column = 0 ; column < n ; column++) {
+            if (canPlace(board, row, column)) {
+                placeQueen(board, row, column);
+                nQueens(row + 1, n, board, queens + 1);
+                removeQueen(board, row, column);
+            }
+        }
+    }
+
+    private static void placeQueen(List<String> board, int row, int column) {
+        board.set(
+                row,
+                board.get(row).substring(0, column) + 'Q' + board.get(row).substring(column + 1)
+        );
+        rows[row] = true;
+        columns[column] = true;
+    }
+
+    private static void removeQueen(List<String> board, int row, int column) {
+        board.set(
+                row,
+                board.get(row).substring(0, column) + '.' + board.get(row).substring(column + 1)
+        );
+        rows[row] = false;
+        columns[column] = false;
+    }
+
+    private static boolean canPlace(List<String> board, int row, int column) {
+        return !rows[row] && !columns[column] && !queenInLeftDiagonal(board, row, column)
+                && !queenInRightDiagonal(board, row, column);
+    }
+
+    private static boolean queenInLeftDiagonal(List<String> board, int row, int column) {
+        for (int i = row - 1, j = column - 1 ; i >= 0 && j >= 0 ; i--, j--) {
+            if (board.get(i).charAt(j) == 'Q') {
+                return true;
+            }
+        }
+        for (int i = row + 1, j = column + 1 ; i < board.size() && j < board.size() ; i++, j++) {
+            if (board.get(i).charAt(j) == 'Q') {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    private static boolean queenInRightDiagonal(List<String> board, int row, int column) {
+        for (int i = row - 1, j = column + 1 ; i >= 0 && j < board.size() ; i--, j++) {
+            if (board.get(i).charAt(j) == 'Q') {
+                return true;
+            }
+        }
+        for (int i = row + 1, j = column - 1 ; i < board.size() && j >= 0 ; i++, j--) {
+            if (board.get(i).charAt(j) == 'Q') {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    private static List<String> getEmptyBoard(int n) {
+        final List<String> board = new ArrayList<>();
+        final String line = ".".repeat(n);
+        for (int i = 0 ; i < n ; i++) {
+            board.add(line);
+        }
+        return board;
+    }
+}