[LeetCode] 51. N-Queens

bool isValid(vector<string>& board, int row, int col) {
// 判断此列上方有没有Queen
for (int i = 0; i < row; i++)
if (board[i][col] == 'Q')
return false;
// 判断右上延伸对角线有没有Queen
for (int i = row - 1, j = col + 1; i >= 0 && j < board[0].size(); i--, j++)
if (board[i][j] == 'Q')
return false;
// 判断左上延伸对角线有没有Queen
for (int i = row - 1, j = col - 1; i >= 0 && j >= 0; i--, j--)
if (board[i][j] == 'Q')
return false;

return true;
}

void dfs(vector<vector<string>>& res, vector<string>& board, int row, int& n) {
if (row == n)
return res.push_back(board);
for (int col = 0; col < n; col++) {
if (isValid(board, row, col)) {
board[row][col] = 'Q';
dfs(res, board, row + 1, n);
board[row][col] = '.';
}
}
}

vector<vector<string>> solveNQueens(int n) {
vector<vector<string>> res;
if (! n) return res;
vector<string> board(n, string(n, '.'));
dfs(res, board, 0, n);
return res;
}