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一、力扣454. 四数相加 II

class Solution {public int fourSumCount(int[] nums1, int[] nums2, int[] nums3, int[] nums4) {Map<Integer, Integer> map = new HashMap<>();int res = 0;for(int i: nums1){for(int j : nums2){int sum = i + j;map.put(sum, map.getOrDefault(sum, 0) + 1);}}for(int i : nums3){for(int j : nums4){res += map.getOrDefault(0-i - j, 0);}}return res;}
}

二、力扣383. 赎金信

class Solution {public boolean canConstruct(String ransomNote, String magazine) {int[] arr = new int[27];for(int i = 0; i < ransomNote.length(); i ++){arr[ransomNote.charAt(i) - 'a'] ++;}for(int i = 0; i < magazine.length(); i ++){arr[magazine.charAt(i) - 'a'] --;}for(int i : arr){if(i > 0){return false;}}return true;}
}

三、力扣15. 三数之和

class Solution {public List<List<Integer>> threeSum(int[] nums) {List<List<Integer>> res = new ArrayList<>();Arrays.sort(nums);for(int i = 0; i < nums.length-2;){if(nums[i] > 0){return res;}int left = i+1, right = nums.length -1;while(left < right){int sum = nums[i] + nums[left] + nums[right];if(sum > 0){right --;}else if(sum < 0){left ++;}else{res.add(Arrays.asList(nums[i], nums[left], nums[right]));while(left < right && nums[left] == nums[left + 1])left ++;while(left < right && nums[right] == nums[right - 1]) right --;left ++;right --;}}while(nums[i] == nums[i+1] && i < nums.length-2)i ++;i ++;}return res;}
}

四、力扣18. 四数之和

class Solution {public List<List<Integer>> fourSum(int[] nums, int target) {List<List<Integer>> res = new ArrayList<>();if(nums.length <= 3){return res;}Arrays.sort(nums);for(int i = 0; i < nums.length -3;){if(target < 0 && nums[i]>0){return res;}for(int j = i + 1; j < nums.length - 2; ){int left = j + 1, right = nums.length - 1;while(left < right){int sum = nums[i] + nums[j] + nums[left] + nums[right];if(sum > target){right --;}else if(sum < target){left ++;}else{res.add(Arrays.asList(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 --;}}while(j < nums.length-2 && nums[j] == nums[j + 1])j ++; j ++;}while(i < nums.length-3 && nums[i] == nums[i+1])i ++; i ++;}return res;}
}