solveNQueens

class Solution {
    public List<List<String>> solveNQueens(int n) {
        char[][] b = new char[n][n];
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                b[i][j] = '.';
            }
        }
        f(0, b,n);
        return res;
    }

    private void f(int r, char[][] b, int n) {
        if (r == n) {
            print(b);
            return;
        }
        for (int c = 0; c < n; c++) {
            if (s(b, r, c, n)) {
                b[r][c] = 'Q';
                f(r + 1, b, n);
                b[r][c] = '.';
            }
        }
    }

    private boolean s(char[][] b, int r, int c, int n) {
        for (int i = 0; i < r; i++) {
            if (b[i][c] == 'Q') return false;
        }
        for (int i = r - 1, j = c - 1; i >= 0 && j >= 0; i--, j--) {
            if (b[i][j] == 'Q') return false;
        }
        for (int i = r - 1, j = c + 1; i >= 0 && j < n; i--, j++) {
            if (b[i][j] == 'Q') return false;
        }
        return true;
    }


}