题目：

题解：

class Solution {public int totalNQueens(int n) {Set<Integer> columns = new HashSet<Integer>();Set<Integer> diagonals1 = new HashSet<Integer>();Set<Integer> diagonals2 = new HashSet<Integer>();return backtrack(n, 0, columns, diagonals1, diagonals2);}public int backtrack(int n, int row, Set<Integer> columns, Set<Integer> diagonals1, Set<Integer> diagonals2) {if (row == n) {return 1;} else {int count = 0;for (int i = 0; i < n; i++) {if (columns.contains(i)) {continue;}int diagonal1 = row - i;if (diagonals1.contains(diagonal1)) {continue;}int diagonal2 = row + i;if (diagonals2.contains(diagonal2)) {continue;}columns.add(i);diagonals1.add(diagonal1);diagonals2.add(diagonal2);count += backtrack(n, row + 1, columns, diagonals1, diagonals2);columns.remove(i);diagonals1.remove(diagonal1);diagonals2.remove(diagonal2);}return count;}}
}