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| 1 | +class Solution { |
| 2 | + |
| 3 | + /** |
| 4 | + * @param Integer $n |
| 5 | + * @return String[][] |
| 6 | + */ |
| 7 | + function solveNQueens($n) { |
| 8 | + $ans = []; |
| 9 | + $colUsed = array_fill(0, $n, false); |
| 10 | + $diagonals45Used = array_fill(0, 2 * $n - 1, false); |
| 11 | + $diagonals135Used = array_fill(0, 2 * $n - 1, false); |
| 12 | + $chars = array_fill(0, $n, array_fill(0, $n, '.')); |
| 13 | + self::helper(0, $chars, $ans, $colUsed, $diagonals45Used, $diagonals135Used); |
| 14 | + return $ans; |
| 15 | + } |
| 16 | + |
| 17 | + function helper($row, &$chars, &$ans, &$colUsed, &$diagonals45Used, &$diagonals135Used){ |
| 18 | + $N = count($chars); |
| 19 | + if ($row == $N) { |
| 20 | + $temp = []; |
| 21 | + for ($i = 0; $i < $N; $i++) |
| 22 | + array_push($temp, implode($chars[$i])); |
| 23 | + array_push($ans, $temp); |
| 24 | + }else{ |
| 25 | + for ($i = 0; $i < $N; $i++){ |
| 26 | + if ($colUsed[$i] || $diagonals45Used[$i+$row] || $diagonals135Used[$N-1-$row+$i]) continue; |
| 27 | + $chars[$row][$i] = 'Q'; |
| 28 | + $colUsed[$i] = $diagonals45Used[$i+$row] = $diagonals135Used[$N-1-$row+$i] = true; |
| 29 | + self::helper($row+1, $chars, $ans, $colUsed, $diagonals45Used, $diagonals135Used); |
| 30 | + $chars[$row][$i] = '.'; |
| 31 | + $colUsed[$i] = $diagonals45Used[$i+$row] = $diagonals135Used[$N-1-$row+$i] = false; |
| 32 | + } |
| 33 | + } |
| 34 | + } |
| 35 | +} |
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