题解 | 正方形的数目全排列暴力搜索

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;

public class Main {
    private static List<Integer> path = new ArrayList<>();
    private static int ans = 0;

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] nums = new int[n];
        for (int i = 0; i < n; i++) nums[i] = sc.nextInt();
        Arrays.sort(nums);
        boolean[] used = new boolean[n];
        backTracking(used, nums);
        System.out.println(ans);
        sc.close();
    }

    private static void backTracking(boolean[] used, int[] nums) {
        if (path.size() == nums.length) {
            ans++;
            return;
        }
        for (int i = 0; i < nums.length; i++) {
            if (used[i] || i > 0 && nums[i] == nums[i - 1] && !used[i - 1]) continue;
            if (!path.isEmpty()) {
                int x = path.get(path.size() - 1) + nums[i];
                if (Math.sqrt(x) % 1 != 0) continue;
            }
            used[i] = true;
            path.add(nums[i]);
            backTracking(used, nums);
            path.remove(path.size() - 1);
            used[i] = false;

        }
    }
}