new soln

Author: shreymehul

51.SolveNQueens.cs

@@ -0,0 +1,63 @@
+// 51. N-Queens
+// The n-queens puzzle is the problem of placing n queens on an n x n chessboard such that no two queens attack each other.
+// Given an integer n, return all distinct solutions to the n-queens puzzle. You may return the answer in any order.
+// Each solution contains a distinct board configuration of the n-queens' placement, where 'Q' and '.' both indicate a queen and an empty space, respectively.
+
+// Example 1:
+// Input: n = 4
+// Output: [[".Q..","...Q","Q...","..Q."],["..Q.","Q...","...Q",".Q.."]]
+// Explanation: There exist two distinct solutions to the 4-queens puzzle as shown above
+// Example 2:
+// Input: n = 1
+// Output: [["Q"]]
+
+// Constraints:
+// 1 <= n <= 9
+
+public class Solution {
+    public IList<IList<string>> SolveNQueens(int n) {
+        List<IList<String>> result = new List<IList<String>>();
+        List<String> board = new List<String>();
+        for(int i = 0; i<n; i++){
+            StringBuilder row = new StringBuilder("");
+            for(int j = 0; j < n; j++)
+                row.Append(".");
+            board.Add(row.ToString());
+        }
+        PlaceQueen(result, board,0);
+        return result;
+    }
+    private void PlaceQueen(List<IList<String>> result, List<String> board,
+        int rowIdx)
+        {
+            if(rowIdx == board.Count){
+                result.Add(new List<String>(board));
+                return;
+            }
+            for(int i = 0; i < board.Count; i++){
+                if(IsSafe(board, rowIdx, i)){
+                    StringBuilder newRow = new StringBuilder(board[rowIdx]);
+                    newRow[i] = 'Q';
+                    board[rowIdx] = newRow.ToString();
+                    PlaceQueen(result, board, rowIdx+1);
+                    newRow[i] = '.';
+                    board[rowIdx] = newRow.ToString();
+                }
+            }
+        }
+    private bool IsSafe(List<String> board, int row, int col){
+        for(int i = row; i >= 0; i--){
+            if(board[i][col] == 'Q')
+                return false;
+        }
+        for(int i = row, j = col; i >= 0 && j >= 0; i--, j--){
+            if(board[i][j] == 'Q')
+                return false;
+        }
+        for(int i = row, j = col; i >= 0 && j < board.Count; i--, j++){
+            if(board[i][j] == 'Q')
+                return false;
+        }
+        return true;
+    }
+}