[Leetcode] n-Queens II

Question: given an integer N, it indicates that the size of the Board is N * n, and N queens are placed on the board. The same row, column, and diagonal lines cannot have the same queen, find the number of feasible solutions

Algorithm: Deep Priority Search + pruning Optimization

The pruning optimization scheme is as follows (take queen 4 as an example ):

It is not difficult to find the following rules:

The same left diagonal line: Row-Col + n = constant
The same right diagonal line: Row + Col = constant value

public class Solution {    final int CHECK_NUMBER = 3;  // check tree points: col, left diagonal and right diagonal    final int MAX_NUMBER = 1024;        int nSolutions = 0;boolean[][] isVisited = new boolean[CHECK_NUMBER][MAX_NUMBER];public int totalNQueens(int n) {        for (int i=0; i<CHECK_NUMBER; ++i) {        isVisited[i] = new boolean[MAX_NUMBER];        }                dfs(0, n);        return nSolutions;    }/** * boolean[0][MAXN]: * * 0: check the col * 1: check the left diagonal * 2. check the right diagonal *  */public void dfs(int row, int nGirds) {if (row == nGirds) {++nSolutions;return ;}for (int col=0; col<nGirds; ++col) {if (!isVisited[0][col]  && !isVisited[1][row-col+nGirds] && !isVisited[2][row+col]) {isVisited[0][col] = true;isVisited[1][row-col+nGirds] = true;isVisited[2][row+col] = true;dfs(row+1, nGirds);isVisited[0][col] = false;isVisited[1][row-col+nGirds] = false;isVisited[2][row+col] = false;}}}}