leetcode 52. N-Queens II

题目内容

Follow up for N-Queens problem.

Now, instead outputting board configurations, return the total number of distinct solutions.

题目分析

n皇后问题，找出所有的可行解。通过本方法，没有记录皇后的坐标

代码

public class Solution {
private final Set occupiedCols = new HashSet();
private final Set occupiedDiag1s = new HashSet();
private final Set occupiedDiag2s = new HashSet();

public int totalNQueens(int n) {
    return totalNQueensHelper(0, 0, n);
}

private int totalNQueensHelper(int row, int count, int n) {
    for (int col = 0; col < n; col++) {
        if (occupiedCols.contains(col))
            continue;
        int diag1 = row - col;
        if (occupiedDiag1s.contains(diag1))
            continue;
        int diag2 = row + col;
        if (occupiedDiag2s.contains(diag2))
            continue;
        // we can now place a queen here
        if (row == n-1)
            count++;
        else {
            occupiedCols.add(col);
            occupiedDiag1s.add(diag1);
            occupiedDiag2s.add(diag2);
            count = totalNQueensHelper(row+1, count, n);
            // recover
            occupiedCols.remove(col);
            occupiedDiag1s.remove(diag1);
            occupiedDiag2s.remove(diag2);
        }
    }

    return count;
}
}