NQueens in CPP

More efficient NQueens solution with a space-time tradeoff to check for safety of position

Author: anishbadhri

BackTracking/NQueens/nqueens.cpp

@@ -0,0 +1,60 @@
+#include <iostream>
+#include <set>
+#include <vector>
+using namespace std;
+set<int> row_c,col_c,mai_c,opp_c;
+void printBoard(vector<vector<int> > board){
+	static int k = 1;
+	cout<<"Board "<<k++<<":\n";
+	for(int i=0;i<board.size();i++){
+		for(int j=0;j<board[i].size();j++){
+			cout<<board[i][j]<<" ";
+		}
+		cout<<endl;
+	}
+	cout<<endl;
+}
+bool isSafe(int row,int col){
+	if(row_c.find(row)!=row_c.end())
+		return false;
+	if(col_c.find(col)!=col_c.end())
+		return false;
+	if(mai_c.find(row-col)!=mai_c.end())
+		return false;
+	if(opp_c.find(row+col)!=opp_c.end())
+		return false;
+	return true;
+}
+void setBoard(vector<vector<int> >&board, int row,int col){
+	board[row][col] = 1;
+	row_c.insert(row);
+	col_c.insert(col);
+	mai_c.insert(row-col);
+	opp_c.insert(row+col);
+}
+void unsetBoard(vector<vector<int> >&board, int row,int col){
+	board[row][col] = 0;
+	row_c.erase(row);
+	col_c.erase(col);
+	mai_c.erase(row-col);
+	opp_c.erase(row+col);	
+}
+void solveBoard(vector<vector<int> > &board,int row){
+	if(row==board.size()){
+		printBoard(board);
+		return;
+	}
+	for(int i=0;i<board.size();i++){
+		if(isSafe(row,i)){
+			setBoard(board,row,i);
+			solveBoard(board,row+1);
+			unsetBoard(board,row,i);
+		}
+	}
+}
+int main(){
+	cout<<"Enter size of board: ";
+	int n; cin>>n;
+	vector<vector<int> > board = vector<vector<int> >(n,std::vector<int> (n,0));
+	solveBoard(board,0);
+}