package jjn.carl.hashtable;
import java.util.HashMap;import java.util.Map;
/** * @author Jiang Jining * @since 2023-07-04 21:33 */public class LeetCode454 {    public int fourSumCount(int[] nums1, int[] nums2, int[] nums3, int[] nums4) {        Map<Integer, Integer> map = new HashMap<>();        int res = 0;        for (int first : nums1) {            for (int second : nums2) {                map.put(first + second, map.getOrDefault(first + second, 0) + 1);            }        }        for (int third : nums3) {            for (int fourth : nums4) {                res += map.getOrDefault(-third - fourth, 0);            }        }        return res;    }        public static void main(String[] args) {        int[] nums1 = new int[]{1, 2};        int[] nums2 = new int[]{-2, -1};        int[] nums3 = new int[]{-1, 2};        int[] nums4 = new int[]{0, 2};        int fourSumCount = new LeetCode454().fourSumCount(nums1, nums2, nums3, nums4);        System.out.println("fourSumCount = " + fourSumCount);    }}

package jjn.carl.hashtable;
/** * @author Jiang Jining * @since 2023-07-04 21:45 */public class LeetCode383 {    public boolean canConstruct(String ransomNote, String magazine) {        int[] charCount = new int[26];        for (char c : ransomNote.toCharArray()) {            charCount[c - 'a']++;        }        for (char c : magazine.toCharArray()) {            charCount[c - 'a']--;        }        for (int j : charCount) {            if (j > 0) {                return false;            }        }        return true;    }        public static void main(String[] args) {        String ransomNote = "aa", magazine = "aab";        boolean canConstruct = new LeetCode383().canConstruct(ransomNote, magazine);        System.out.println("canConstruct = " + canConstruct);    }}

package jjn.carl.hashtable;
import java.util.ArrayList;import java.util.Arrays;import java.util.List;
/** * @author Jiang Jining * @since 2023-07-04 22:13 */public class LeetCode15 {    public List<List<Integer>> threeSum(int[] nums) {        Arrays.sort(nums);        List<List<Integer>> result = new ArrayList<>();        for (int i = 0; i < nums.length - 2; i++) {            if (nums[i] > 0) {                return result;            }            if (i > 0 && nums[i] == nums[i - 1]) {                continue;            }            int left = i + 1, right = nums.length - 1;            while (left < right) {                if (nums[left] + nums[right] + nums[i] < 0) {                    left++;                } else if (nums[left] + nums[right] + nums[i] > 0) {                    right--;                } else {                    result.add(List.of(nums[i], nums[left], nums[right]));                    while (left < right && nums[right] == nums[right - 1]) {                        right--;                    }                    while (left < right && nums[left] == nums[left + 1]) {                        left++;                    }                    left++;                    right--;                }            }        }        return result;    }        public static void main(String[] args) {        int[] nums = new int[]{-1, 0, 1, 2, -1, -4};        List<List<Integer>> threeSum = new LeetCode15().threeSum(nums);        System.out.println("Arrays.toString(threeSum) = " + threeSum.toString());    }}

package jjn.carl.hashtable;
import java.util.ArrayList;import java.util.Arrays;import java.util.List;
/** * @author Jiang Jining * @since 2023-07-04 23:10 */public class LeetCode18 {    public List<List<Integer>> fourSum(int[] nums, int target) {        // 排序很关键,是双指针法的前提        Arrays.sort(nums);        long res = 0L;        List<List<Integer>> result = new ArrayList<>();        for (int i = 0; i < nums.length - 3; i++) {            // target 可以为负数,负数相加会变得更小            if (nums[i] > 0 && nums[i] > target && target >= 0) {                return result;            }            if (i > 0 && nums[i] == nums[i - 1]) {                continue;            }            for (int j = i + 1; j < nums.length; j++) {                if (j > i + 1 && nums[j] == nums[j - 1]) {                    continue;                }                int left = j + 1, right = nums.length - 1;                while (left < right) {                    // 防止加法越界                    long cur = (long) nums[i] + (long) nums[j] + (long) nums[left] + (long) nums[right];                    if (cur < target) {                        left++;                    } else if (cur > target) {                        right--;                    } else {                        result.add(List.of(nums[i], nums[j], nums[left], nums[right]));                        while (left < right && nums[left] == nums[left + 1]) {                            left++;                        }                        while (left < right && nums[right] == nums[right - 1]) {                            right--;                        }                        left++;                        right--;                    }                }            }        }        return result;    }        public static void main(String[] args) {        int[] nums = new int[]{1, 0, -1, 0, -2, 2};        int target = 0;        List<List<Integer>> fourSum = new LeetCode18().fourSum(nums, target);        System.out.println("fourSum = " + fourSum);                // Case two        nums = new int[]{2, 2, 2, 2, 2, 2};        target = 8;        fourSum = new LeetCode18().fourSum(nums, target);        System.out.println("fourSum = " + fourSum);              // 非常猥琐的测试用例        nums = new int[]{1000000000, 1000000000, 1000000000, 1000000000};        target = -294967296;        fourSum = new LeetCode18().fourSum(nums, target);        System.out.println("fourSum = " + fourSum);    }}