题目

class Solution {public List<List<String>> solveNQueens(int n) {List<List<String>> res = new ArrayList<>();char[][] tmp = new char[n][n];for (int i = 0; i < n; ++i) {Arrays.fill(tmp[i], '.');}dfs(tmp, 0, res);return res;}public void dfs(char[][] tmp, int layer, List<List<String>> res) {int n = tmp.length;if (layer == n) {List<String> solu = new ArrayList<>();for (int k = 0; k < n; ++k) {solu.add(String.valueOf(tmp[k]));}res.add(solu);return;}for (int k = 0; k < n; ++k) {if (isValid(tmp, layer, k)) {tmp[layer][k] = 'Q';dfs(tmp, layer + 1, res);tmp[layer][k] = '.';}}}boolean isValid(char[][] tmp, int x, int y) {int n = tmp.length;for (int i = 0; i < n; ++i) {if (tmp[i][y] == 'Q' && i < x) {return false;}}int i = 1;while (x - i >= 0 && y - i >= 0) { if (tmp[x - i][y - i] == 'Q') {return false;}++i;}i = 1;while (x - i >= 0 && y + i < n) {if (tmp[x - i][y + i] == 'Q') {return false;}++i;}return true;}
}