added a condition for n<3 as a solution does not exist

CPP/recursion/N_Queens/N_Queens.cpp

@@ -2,13 +2,18 @@
 #include <vector>
 
 // Function to print the bord
-void printBoard(std::vector<std::vector<int>>& board, int n) {
-    for (int i = 0; i < n; ++i) {
-        for (int j = 0; j < n; ++j) {
-            if (board[i][j] == 1) {
+void printBoard(std::vector<std::vector<int>> &board, int n)
+{
+    for (int i = 0; i < n; ++i)
+    {
+        for (int j = 0; j < n; ++j)
+        {
+            if (board[i][j] == 1)
+            {
                 std::cout << 'Q' << ' ';
             }
-            else {
+            else
+            {
                 std::cout << '_' << ' ';
             }
         }
@@ -17,11 +22,14 @@ void printBoard(std::vector<std::vector<int>>& board, int n) {
 }
 
 // Function for checking valid squares.
-bool canBePlaced(std::vector<std::vector<int>>& board, int i, int j, int  n) {
+bool canBePlaced(std::vector<std::vector<int>> &board, int i, int j, int n)
+{
 
     // Vertical check.
-    for (int v = 0; v < i; ++v) {
-        if (board[v][j] == 1) {
+    for (int v = 0; v < i; ++v)
+    {
+        if (board[v][j] == 1)
+        {
             return false;
         }
     }
@@ -30,17 +38,21 @@ bool canBePlaced(std::vector<std::vector<int>>& board, int i, int j, int  n) {
     int u = i, v = j;
 
     // Upper left diagonal check.
-    while (i >= 0 && j >= 0) {
-        if(board[i][j] == 1) {
+    while (i >= 0 && j >= 0)
+    {
+        if (board[i][j] == 1)
+        {
             return false;
         }
         --i;
         --j;
     }
 
     // Upper right diagonal.
-    while (u >= 0 && v < n) {
-        if (board[u][v] == 1) {
+    while (u >= 0 && v < n)
+    {
+        if (board[u][v] == 1)
+        {
             return false;
         }
         --u;
@@ -52,11 +64,13 @@ bool canBePlaced(std::vector<std::vector<int>>& board, int i, int j, int  n) {
 }
 
 // Main recursive function.
-bool N_Queens(std::vector<std::vector<int>>& board, int n, int row) {
+bool N_Queens(std::vector<std::vector<int>> &board, int n, int row)
+{
     // --> base case
     // When row becomes equal to n
     // means all queens have been placed in the right manner.
-    if (row == n) {
+    if (row == n)
+    {
         // Print the first possible way.
         printBoard(board, n);
         // Endl for printing the next possible board
@@ -68,20 +82,24 @@ bool N_Queens(std::vector<std::vector<int>>& board, int n, int row) {
     }
 
     // --> recursive case
-    for (int column = 0; column < n; ++column) {
+    for (int column = 0; column < n; ++column)
+    {
         // Check if the queen can be placed at the ith row and jth column.
-        if (canBePlaced(board, row, column, n)) {
+        if (canBePlaced(board, row, column, n))
+        {
             // Mark that place as 1.
             board[row][column] = 1;
             // Check for further positions.
             bool remaining_positions = N_Queens(board, n, row + 1);
-            if (remaining_positions) {
+            if (remaining_positions)
+            {
                 // If queens can be placed in the remaining positions
                 // it means the placing of queens can proceed further.
                 // return true
                 return true;
             }
-            else {
+            else
+            {
                 // If queens cannot be placed further in the right way
                 // backtrack to the previous position.
                 board[row][column] = 0;
@@ -92,18 +110,22 @@ bool N_Queens(std::vector<std::vector<int>>& board, int n, int row) {
     return false;
 }
 
-int main() {
+int main()
+{
 
     int n = 0;
     // Input Size of the chessboard
     // No of queens will be the same as size n.
     std::cin >> n;
-
+    if (n <= 3)
+    {
+        std::cout << "No solution exists for n = " << n << std::endl;
+    }
     // 2D vector for storing board having size max size of nXn.
     // First create a column vector of size n.
     std::vector<int> column(n);
     // Create Row vectors and fill rows with columns.
-    std::vector<std::vector<int>> board(n,column);
+    std::vector<std::vector<int>> board(n, column);
 
     // Function call for
     // Arguments passed are : 2D vector board