题目：

题解：

class Solution {
public:vector<vector<string>> solveNQueens(int n) {auto solutions = vector<vector<string>>();auto queens = vector<int>(n, -1);auto columns = unordered_set<int>();auto diagonals1 = unordered_set<int>();auto diagonals2 = unordered_set<int>();backtrack(solutions, queens, n, 0, columns, diagonals1, diagonals2);return solutions;}void backtrack(vector<vector<string>> &solutions, vector<int> &queens, int n, int row, unordered_set<int> &columns, unordered_set<int> &diagonals1, unordered_set<int> &diagonals2) {if (row == n) {vector<string> board = generateBoard(queens, n);solutions.push_back(board);} else {for (int i = 0; i < n; i++) {if (columns.find(i) != columns.end()) {continue;}int diagonal1 = row - i;if (diagonals1.find(diagonal1) != diagonals1.end()) {continue;}int diagonal2 = row + i;if (diagonals2.find(diagonal2) != diagonals2.end()) {continue;}queens[row] = i;columns.insert(i);diagonals1.insert(diagonal1);diagonals2.insert(diagonal2);backtrack(solutions, queens, n, row + 1, columns, diagonals1, diagonals2);queens[row] = -1;columns.erase(i);diagonals1.erase(diagonal1);diagonals2.erase(diagonal2);}}}vector<string> generateBoard(vector<int> &queens, int n) {auto board = vector<string>();for (int i = 0; i < n; i++) {string row = string(n, '.');row[queens[i]] = 'Q';board.push_back(row);}return board;}
};