Create 51-N-Queens.java

Author: SharmaTushar1

java/51-N-Queens.java

@@ -0,0 +1,57 @@
+class Solution {
+    public List<List<String>> solveNQueens(int n) {
+        List<List<String>> ans = new ArrayList<List<String>>();
+        boolean[][] board = new boolean[n][n];
+        queens(board, 0, ans);
+        return ans;
+        
+    }
+    
+    public void queens(boolean[][] board, int row , List<List<String>> ans2) {
+        //base case
+        if (row == board.length) {
+            ArrayList<String> ans = new ArrayList<String>();
+            createAnswer(board, ans);
+            ans2.add(ans);
+            return;
+        }
+        for (int col = 0; col<board.length; col++) {
+            if (isSafe(board,row, col)) {
+                board[row][col] = true;
+                queens(board, row+1, ans2);
+                board[row][col] = false;
+            }
+        }
+    }
+    
+    public void createAnswer(boolean[][] board, ArrayList<String> ans) {
+        for (int i = 0; i<board.length; i++) {
+            StringBuilder str = new StringBuilder();
+            for (int j = 0; j<board[0].length; j++) {
+                if (board[i][j]) {
+                    str.append("Q");
+                } else str.append(".");
+            }
+            ans.add(str.toString());
+        }
+    }
+    
+    public boolean isSafe(boolean[][] board, int row, int col) {
+        for (int i = 0; i<row; i++) {
+            if (board[i][col]) {
+                return false;
+            }
+        }
+        int maxLeft = Math.min(row,col);
+        for (int i = 1; i<=maxLeft; i++) {
+            if (board[row-i][col-i]) {
+                return false;
+            }
+        }
+        int maxRight = Math.min(row,board.length-1-col);
+        for (int i = 1; i<=maxRight; i++) {
+            if (board[row-i][col+i]) return false;
+        }
+        return true;
+    }
+}