Create NQueens.cpp

Added Solution for the Problem NQueens.

leetcode/NQueens.cpp

@@ -0,0 +1,76 @@
+// N Queens Problem - Leetcode
+// https://leetcode.com/problems/n-queens/
+class Solution
+{
+public:
+    bool isSafe(int n, int row, int col, vector<string> board)
+    {
+        int dr = row;
+        int dc = col;
+        while (row >= 0 && col >= 0)
+        {
+            if (board[row][col] == 'Q')
+            {
+                return false;
+            }
+            row--;
+            col--;
+        }
+        col = dc;
+        row = dr;
+        while (col >= 0)
+        {
+            if (board[row][col] == 'Q')
+            {
+                return false;
+            }
+            col--;
+        }
+        row = dr;
+        col = dc;
+        while (row < n && col >= 0)
+        {
+            if (board[row][col] == 'Q')
+            {
+                return false;
+            }
+            row++;
+            col--;
+        }
+
+        return true;
+    }
+    void solve(int n, int col, vector<string> &board, vector<vector<string>> &ans)
+    {
+        if (col == n)
+        {
+            ans.push_back(board);
+            return;
+        }
+
+        for (int row = 0; row < n; row++)
+        {
+            if (isSafe(n, row, col, board))
+            {
+                board[row][col] = 'Q';
+
+                solve(n, col + 1, board, ans);
+
+                board[row][col] = '.';
+            }
+        }
+    }
+    vector<vector<string>> solveNQueens(int n)
+    {
+        vector<vector<string>> ans;
+        vector<string> board(n);
+        string s(n, '.');
+        for (int i = 0; i < n; i++)
+        {
+            board[i] = s;
+        }
+        solve(n, 0, board, ans);
+
+        return ans;
+    }
+};