N-Queen Problem in Java Language

Java/Backtracking/N-QueenProblem.java

@@ -0,0 +1,46 @@
+import java.util.*;
+
+// N-Queen Problem
+
+class Queen {
+    private boolean canPlace(char[][] board, int row, int col, int n) {
+        // upper-left diagonal
+        for (int r = row, c = col; r >= 0 && c >= 0; r--, c--) 
+            if (board[r][c] == 'Q') return false;
+
+        // same row (left)
+        for (int c = col; c >= 0; c--) 
+            if (board[row][c] == 'Q') return false;
+
+        // bottom-left diagonal
+        for (int r = row, c = col; r < n && c >= 0; r++, c--) 
+            if (board[r][c] == 'Q') return false;
+
+        return true;
+    }
+
+    private void placements(int col, char[][] board, List<List<String>> ans, int n) {
+        if (col == n) {
+            List<String> temp = new ArrayList<>();
+            for (int i = 0; i < n; i++) temp.add(new String(board[i]));
+            ans.add(temp);
+            return;
+        }
+
+        for (int row = 0; row < n; row++) {
+            if (canPlace(board, row, col, n)) {
+                board[row][col] = 'Q';
+                placements(col + 1, board, ans, n);
+                board[row][col] = '.';
+            }
+        }
+    }
+
+    public List<List<String>> solveNQueens(int n) {
+        List<List<String>> ans = new ArrayList<>();
+        char[][] board = new char[n][n];
+        for (int i = 0; i < n; i++) Arrays.fill(board[i], '.');
+        placements(0, board, ans, n);
+        return ans;
+    }
+}