N皇后问题

public class Solution {

    public static List<List<String>> solveNQueens(int n) {
        List<List<String>> res = new ArrayList<>();
        int[] queenList = new int[n];
        placeQueen(queenList, 0, n, res); //在第0行放Q
        return res;
    }

    private static void placeQueen(int[] queenList, int row, int n, List<List<String>> res) {
        // 已经填满，生成结果
        if(row == n) {
            ArrayList<Stirng> list = new ArrayList<>();
            for(int i = 0; i < n; i++) {

                String str = "";
                for(int col = 0; col < n; col++) {
                    if(queenList[i] == col) {
                        str+="Q";
                    } else {
                        str+=".";
                    }
                }

                list.add(str);
            }

            res.add(list);
        }

        for(int col = 0; col < n; col++) {
            if(isValid(queenList, row, col)) {
                // important here
                queenList[row] == col;

                placeQueen(queenList, row+1, n, res);
            }
        }
    }

    private static boolean isValid(int[] queenList, int row, int col) {
        for (int i = 0; i < row; i++) {

            int pos = queenList[i];

            if (pos == col) { //和新加入的Q处于同一列
                return false;
            }

            if (pos + row - i == col) { //在新加入的Q的右对角线上
                return false;
            }

            if (pos - row + i == col) { //在新加入的Q的左对角线上
                return false;
            }
        }

        return true;
    }
}