输入:1
输出:[["Q"]]

输入:4
输出:
[// Solution 1[".Q..","...Q","Q...","..Q."],// Solution 2["..Q.","Q...","...Q",".Q.."]
]

class Solution {/*** Get all distinct N-Queen solutions* @param n: The number of queens* @return: All distinct solutions* For example, A string '...Q' shows a queen on forth position*/List<List<String>> solveNQueens(int n) {// result用于存储答案List<List<String>> results = new ArrayList<>();if (n <= 0) {return results;}search(results, new ArrayList<Integer>(), n);return results;}// search函数为搜索函数，n表示已经放置了n个皇后，cols 表示每个皇后所在的列private void search(List<List<String>> results, List<Integer> cols, int n) {// 若已经放置了n个皇后表示出现了一种解法，绘制后加入答案resultif (cols.size() == n) {results.add(Draw(cols));return;}// 枚举当前皇后放置的列，若不合法则跳过for (int colIndex = 0; colIndex < n; colIndex++) {if (!isValid(cols, colIndex)) {continue;}// 若合法则递归枚举下一行的皇后cols.add(colIndex);search(results, cols, n);cols.remove(cols.size() - 1);}}// isValid函数为合法性判断函数private boolean isValid(List<Integer> cols, int col) {int row = cols.size();for (int rowIndex = 0; rowIndex < cols.size(); rowIndex++) {//若有其他皇后在同一列或同一斜线上则不合法if (cols.get(rowIndex) == col) {return false;}if (row + col == rowIndex + cols.get(rowIndex)) {return false;}if (row - col == rowIndex - cols.get(rowIndex)) {return false;}}return true;}// Draw函数为将 cols 数组转换为答案的绘制函数private List<String> Draw(List<Integer> cols) {List<String> result = new ArrayList<>();for (int i = 0; i < cols.size(); i++) {StringBuilder sb = new StringBuilder();for (int j = 0; j < cols.size(); j++) {sb.append(j == cols.get(i) ? 'Q' : '.');}result.add(sb.toString());}return result;}
}