Implement N Queens Solution Dart

README.md

@@ -375,6 +375,7 @@ Check out ---> [Sample PR](https://github.com/codedecks-in/LeetCode-Solutions/pu
 | 980 | [Unique Paths III](https://leetcode.com/problems/unique-paths-iii/) | [C++](./C++/Unique-Paths-III.cpp) | _O(R * C * 2 ^ (R \* C))_ | _O(R \* C)_ | Hard       | DFS, Memoization    |      |
 | 39  | [Combination Sum](https://leetcode.com/problems/combination-sum/)   | [C++](./C++/combination-sum.cpp)  | _O(2^n)_                  | _O(n)_      | Medium     | Array, Backtracking |      |
 | 17  | [Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/)   | [C++](./C++/letter-combinations-of-a-phone-number.cpp)  | _O(4^n)_                  | _O(n)_      | Medium     | String, Hash Table, Backtracking |      |
+| 051 | [N Queens](https://leetcode.com/problems/n-queens/description/)       | [Dart](./dart/n_queens.dart)    | _O(n!)_                  | _O(n)_      | Hard       | Array, Backtracking          |      |
 
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dart/n_queens.dart

@@ -0,0 +1,61 @@
+/*
+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.
+*/
+
+class Solution {
+  List<List<String>> solveNQueens(int n) {
+    final List<List<String>> board = List.generate(n, (_) => List<String>.filled(n, '.'));
+    final List<List<String>> ans = [];
+    backtrack(board, ans, 0, 0, n);
+    return ans;
+  }
+
+  bool isSafe(List<List<String>> board, int row, int col) {
+    for (int i = 0; i < row; 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.length; i--, j++) {
+      if (board[i][j] == 'Q') {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  void backtrack(List<List<String>> board, List<List<String>> ans, int row, int col, int queens) {
+    if (row == queens) {
+      ans.add(convert(board));
+      return;
+    }
+
+    for (int col = 0; col < board.length; col++) {
+      if (isSafe(board, row, col)) {
+        board[row][col] = 'Q';
+        backtrack(board, ans, row + 1, col, queens);
+        board[row][col] = '.';
+      }
+    }
+  }
+
+  List<String> convert(List<List<String>> board) {
+    final List<String> ans = [];
+    for (int i = 0; i < board.length; i++) {
+      final row = board[i].join('');
+      ans.add(row);
+    }
+
+    return ans;
+  }
+}