1 public class Solution {
 2     int res = 0; // res should be global, cause java func cannot return int value back.
 3     public int totalNQueens(int n) {
 4         //int res = 0;
 5         //int[] col4row = new int[n];
 6         helper(n, 0, new int[n]);
 7         return res;
 8     }
 9     
10     private void helper(int n, int row, int[] col4row) {
11         if(row == n) { // all positions are ok
12             ++res;
13             return; // do not forget to quit from this recursion here.
14         }
15         for(int i = 0; i < n; i++) {
16             col4row[row] = i; // put every col to this row to test the correctness.
17             if(check(row, col4row)) {
18                 helper(n, row+1, col4row);
19             }
20         }
21     }
22     
23     private boolean check(int row, int[] col4row) {
24         for(int i = 0; i < row; i++) {
25             if(col4row[i] == col4row[row] ||
26                 Math.abs(col4row[i]-col4row[row]) == row-i)
27                 return false;
28         }
29         return true;
30     }
31 }

public class Solution {
    public int totalNQueens(int n) {
        return solve(0, n, new int[n]);
    }

    private int solve(int row, int n, int[] col4row) {
        if (row == n) return 1;
        int ans = 0;
        for (int i = 0; i < n; i++) {
            boolean conflict = false;
            for (int j = 0; j < row; j++) // check first, if not ok then go to next col, if ok let col4row[row] = i;
                if (i == col4row[j] || row - j == Math.abs(i - col4row[j]))
                    conflict = true;
            if (conflict) continue;
            col4row[row] = i;
            ans += solve(row + 1, n, col4row); // this row is finished, go to next row
        }
        return ans;
    }
}