Create 51. N-Queens

Author: khushi3004

51. N-Queens

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+class Solution {
+    List<List<String>> result;
+    public List<List<String>> solveNQueens(int n) {
+        result = new ArrayList<>();
+        //Take a board. .Q
+        char[][] board = new char[n][n];
+        for(int i =0; i < n; i++) {
+            for(int j = 0; j < n; j++) {
+                board[i][j] = '.';
+            }
+        }
+        List<int[]> queenPos = new ArrayList<>();
+        dfs(board, 0, queenPos);
+        return result;
+    }
+    
+    private void dfs(char[][] board, int row, List<int[]> queenPos) {
+        //Chaeck if we have placed all the queens
+        if(queenPos.size() == board.length) {
+            //Construct the result
+            List<String> rowData = new ArrayList<>();
+            for(char[] c : board){
+                rowData.add(new String(c));
+            }
+            result.add(rowData);
+        }
+        
+        //Try adduing Queen at row, col
+        for(int col = 0; col < board.length; col++) {
+            if(canAddQueen(queenPos, row, col)) {
+                board[row][col] = 'Q';
+                queenPos.add(new int[]{row, col});
+                dfs(board, row+1, queenPos);
+                board[row][col] = '.';
+                queenPos.remove(queenPos.size()-1);
+            }
+        }
+    }
+    
+    private boolean canAddQueen(List<int[]> queenPos, int row, int col) {
+        for(int[] pair : queenPos) {
+            int dx = Math.abs(pair[0]- row);
+            int dy = Math.abs(pair[1]- col);
+            if(dx==0 || dy == 0 || dx == dy) return false;
+        }
+        return true;
+    }
+}