2022.2.13 LeetCode —— 哈希表

一、今日刷题

1. 第六部分：哈希表 -- 454.四数相加 II

跳转LeetCode

答案代码：

分组 + 哈希表

package HashSet;

import java.util.HashMap;
import java.util.Map;


public class FourSumCount {
    public static void main(String[] args) {
        int[] nums1 = {1, 2};
        int[] nums2 = {-2, -1};
        int[] nums3 = {-1, 2};
        int[] nums4 = {0, 2};
        FourSumCount fourSumCount = new FourSumCount();
        int ans = fourSumCount.fourSumCount(nums1, nums2, nums3, nums4);
        System.out.println(ans);
    }

    public int fourSumCount(int[] nums1, int[] nums2, int[] nums3, int[] nums4) {
        Map map = new HashMap<>();
        int count = 0;
        for (int num1 : nums1) {
            for (int num2 : nums2) {
                int addNumber = num1 + num2;
                int frequency = map.getOrDefault(addNumber, 0);
                map.put(addNumber, frequency + 1);
            }
        }
        for (int num3 : nums3) {
            for (int num4 : nums4) {
                int checkNumber = -(num3 + num4);
                if (map.containsKey(checkNumber)) {
                    int addCount = map.get(checkNumber);
                    count += addCount;
                }
            }
        }
        return count;
    }
}