algorithms/nQueens/nQueuens.II.cpp

// Source : https://oj.leetcode.com/problems/n-queens-ii/
// Author : Hao Chen
// Date   : 2014-08-22

/********************************************************************************** 
* 
* Follow up for N-Queens problem.
* 
* Now, instead outputting board configurations, return the total number of distinct solutions.
* 
*               
**********************************************************************************/

#include <stdlib.h>
#include <iostream>
#include <vector>
#include <string>
using namespace std;

int totalNQueens(int n); 
void solveNQueensRecursive(int n, int currentRow, vector<int>& solution, int& result); 
bool isValid(int attemptedColumn, int attemptedRow, vector<int> &queenInColumn); 


int totalNQueens(int n) {
    int result=0;
    vector<int> solution(n);

    solveNQueensRecursive(n, 0, solution, result);

    return result;    
}

// the solution is same as the "N Queens" problem.
void solveNQueensRecursive(int n, int currentRow, vector<int>& solution, int& result) {

    for (int i = 0; i < n; i++) {
        if (isValid(i, currentRow, solution) ) {
            if (currentRow+1 == n){
                result++;
                continue;
            }
            solution[currentRow] = i;
            solveNQueensRecursive(n, currentRow+1, solution, result);
        }
    } 
}


bool isValid(int attemptedColumn, int attemptedRow, vector<int> &queenInColumn) {

    for(int i=0; i<attemptedRow; i++) {
        if (attemptedColumn == queenInColumn[i]  || 
            abs(attemptedColumn - queenInColumn[i]) == abs(attemptedRow - i)) {
            return false;
        } 
    }
    return true;
}


int main(int argc, char** argv)
{
    int n = 8;
    if (argc>1){
        n = atoi(argv[1]);
    }
    
    int result = totalNQueens(n);
    
    cout << n << " Queens, total = " << result << endl;
    
    return 0;
}