LeetCode-N-Queens II

Follow up for N-Queens problem.

Now, instead outputting board configurations, return the total number of distinct solutions.

Solution:

Code:

<span style="font-size:14px;">class Solution {
public:
    void helper(vector<int> &positions, const int &n, int index, int &result) {
        if (index == n) {
            ++result;
            return;
        }
        for (int i = 0; i < n; ++i) {
            bool invalid = false;
            for (int j = 0; j < index; ++j)
                if (positions[j] == i || abs(positions[j]-i) == abs(j-index)) {
                    invalid = true;
                    break;
                }
            if (!invalid) {
                positions[index] = i;
                helper(positions, n, index+1, result);
            }
        } 
    }
    
    int totalNQueens(int n) {
        if (n == 0 || n == 1) return n;
        int result = 0;
        vector<int> positions(n, -1);
        helper(positions, n, 0, result);
        return result;
    }
};</span>