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n-queens.java
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n-queens.java
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//
//
package java;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
class NQueens {
List<List<String>> ans = new ArrayList<>();
boolean[] visitedRow;
boolean[] visitedCol;
boolean[] diagonal;
boolean[] backDiagonal;
public List<List<String>> solveNQueens(int n) {
int[] queens = new int[n];
visitedRow = new boolean[n];
visitedCol = new boolean[n];
diagonal = new boolean[2 * n];
backDiagonal = new boolean[2 * n];
backtrace(queens, n, 0);
return ans;
}
public void backtrace(int[] queens, int n, int row) {
if (row == n) {
ans.add(generateBoard(queens, n));
return;
}
for (int col = 0; col < n; col++) {
if (visitedRow[row] || visitedCol[col] || diagonal[row + col] || backDiagonal[row - col + n - 1]) {
continue;
}
queens[row] = col;
visitedRow[row] = true;
visitedCol[col] = true;
diagonal[row + col] = true;
backDiagonal[row - col + n - 1] = true;
backtrace(queens, n, row + 1);
queens[row] = 0;
visitedRow[row] = false;
visitedCol[col] = false;
diagonal[row + col] = false;
backDiagonal[row - col + n - 1] = false;
}
}
public List<String> generateBoard(int[] queens, int n) {
List<String> board = new ArrayList<>();
char[] row = new char[n];
for (int i = 0; i < n; i++) {
Arrays.fill(row, '.');
row[queens[i]] = 'Q';
board.add(new String(row));
}
return board;
}
}