The n-queens puzzle is the problem of placing n queens on an n x n chessboard such that no two queens attack each other.
Given an integer n, return the number of distinct solutions to the n-queens puzzle.
class Solution {
public:
vector<vector<string>> solveNQueens(int n) {
vector < vector < string >> ans;
vector < string > board(n);
string s(n, '.');
for (int i = 0; i < n; i++) {
board[i] = s;
}
dfs(0, board, ans, n);
return ans;
}
void dfs(int col, vector < string > & board, vector < vector < string >> & ans, int n) {
if (col == n) {
ans.push_back(board);
return;
}
for (int row = 0; row < n; row++) {
if (isSafe(row, col, board, n)) {
board[row][col] = 'Q';
dfs(col + 1, board, ans, n);
board[row][col] = '.';
}
}
}
bool isSafe(int row, int col, vector < string > board, int n) {
// check upper element
int duprow = row;
int dupcol = col;
while (row >= 0 && col >= 0) {
if (board[row][col] == 'Q')
return false;
row--;
col--;
}
col = dupcol;
row = duprow;
while (col >= 0) {
if (board[row][col] == 'Q')
return false;
col--;
}
row = duprow;
col = dupcol;
while (row < n && col >= 0) {
if (board[row][col] == 'Q')
return false;
row++;
col--;
}
return true;
}
};
