Create 51. N-Queens.md

Author: TBXark

51. N-Queens.md

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+# 51. N-Queens
+
+### 2020-07-29
+
+The *n*-queens puzzle is the problem of placing *n* queens on an *n*×*n* chessboard such that no two queens attack each other.
+
+![img](https://assets.leetcode.com/uploads/2018/10/12/8-queens.png)
+
+
+# Solution
+
+```swift
+ 
+class Solution {
+      
+   private func checkLine(y: Int, n: Int, col: inout [Bool], row: inout [Int], d1: inout [Bool], d2: inout [Bool], res: inout [[String]]) {
+        if y == n {
+            var temp = [String]()
+            for i in 0..<n {
+                var line = Array<Character>.init(repeating: ".", count: n)
+                line[row[i]] = "Q"
+                temp.append(String(line))
+            }
+            res.append(temp)
+        } else {
+            for x in 0..<n {
+                if !col[x], !d1[x + y], !d2[x - y + (n - 1)] {
+                    col[x] = true
+                    d1[x+y] = true
+                    d2[x - y + (n - 1)] = true
+                    row[y] = x
+                    checkLine(y: y + 1, n: n, col: &col, row: &row, d1: &d1, d2: &d2, res: &res)
+                    col[x] = false
+                    d1[x+y] = false
+                    d2[x - y + (n - 1)] = false
+                }
+            }
+        }
+    }
+    
+    func solveNQueens(_ n: Int) -> [[String]] {
+        var res = [[String]]()
+        guard n > 0 else {
+            return res
+        }
+        var col = [Bool].init(repeating: false, count: n)
+        var row = [Int].init(repeating: 0, count: n)
+        var d1 = [Bool].init(repeating: false, count: (2 * n ) - 1)
+        var d2 = [Bool].init(repeating: false, count: (2 * n ) - 1)
+        checkLine(y: 0, n: n, col: &col, row: &row, d1: &d1, d2: &d2, res: &res)
+        return res
+    }
+}
+
+```