LeetCode 51 N皇后(java)

class Solution {
     
    // 结果集
    private List<List<String>> res = new ArrayList<>();
    // 空白格
    private String empty;

    public List<List<String>> solveNQueens(int n) {
     
        StringBuilder sb = new StringBuilder();
        // 字符串追加
        for (int i = 0; i < n; i++) {
     
            sb.append('.');
        }
        this.empty = sb.toString();
        // 回溯, 返回结果集
        backtrack(new ArrayList<>(), n);
        return res;
    }

    private void backtrack(List<String> acc, int n) {
     
        // 终止条件
        if (acc.size() == n) {
     
            // 查询结果条件
            res.add(acc);
            return;
        }
        // 处理当前层逻辑
        boolean[] taken = new boolean[n];
        for (int i = 0; i < acc.size(); i++) {
     
            // 对角棋子
            int delta = acc.size() - i;
            String prev = acc.get(i);
            for (int j = 0; j < n; j++) {
     
                // 确认皇后位置,结束条件
                if (prev.charAt(j) == 'Q') {
     
                    taken[j] = true;
                    if (j + delta < n) {
     
                        taken[j + delta] = true;
                    }
                    if (j - delta >= 0) {
     
                        taken[j - delta] = true;
                    }
                    break;
                }
            }
        }

        StringBuilder sb = new StringBuilder(empty);
        // 遍历
        for (int i = 0; i < n; i++) {
     
            if (!taken[i]) {
     
                sb.setCharAt(i, 'Q');
                List<String> next = new ArrayList<>(acc);
                next.add(sb.toString());
                // 递归调用下一层
                backtrack(next, n);
                sb.setCharAt(i, '.');
            }
        }
    }
}