代码随想录训练营第三十期|第七天|哈希表part02|454.四数相加II ● 383. 赎金信 ● 15. 三数之和 ● 18. 四数之和 ● 总结

454. 四数相加 II - 力扣（LeetCode）

class Solution {
    public int fourSumCount(int[] nums1, int[] nums2, int[] nums3, int[] nums4) {
        Map<Integer, Integer> map = new HashMap<>();

        for (int n1 : nums1) {
            for (int n2 : nums2) {
                int sum = n1 + n2;
                map.put(sum, map.getOrDefault(sum, 0) + 1);
            }
        }

        int res = 0;
        for (int n3 : nums3) {
            for (int n4 : nums4) {
                int remain = 0 - n3 - n4;
                res += map.getOrDefault(remain, 0);
            }
        }
        return res;
    }
}

383. 赎金信 - 力扣（LeetCode）

class Solution {
    public boolean canConstruct(String ransomNote, String magazine) {
        if (ransomNote.length() > magazine.length()) return false;
        
        int[] count = new int[26];
        for (int i = 0; i < magazine.length(); i++) {
            count[magazine.charAt(i) - 'a']++;
        }

        for (int i = 0; i < ransomNote.length(); i++) {
            count[ransomNote.charAt(i) - 'a']--;
        }

        for (int n : count) {
            if (n < 0) return false;
        }

        return true;
    }
}

15. 三数之和 - 力扣（LeetCode）

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        if (nums.length < 3) return res;
        Arrays.sort(nums);

        for (int i = 0; i < nums.length; i++) {
            if (i > 0 && nums[i] == nums[i - 1]) continue;

            int left = i + 1;
            int right = nums.length - 1;

            while (left < right) {
                int sum = nums[i] + nums[left] + nums[right];
                if (sum < 0) {
                    left++;
                } else if (sum > 0) {
                    right--;
                } 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--;
                }
            }
        }
        return res;
    }
}

18. 四数之和 - 力扣（LeetCode）

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> res = new ArrayList<>();
        Arrays.sort(nums);

        for (int i = 0; i < nums.length; i++) {
            if (nums[i] > 0 && nums[i] > target) return res;
            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;
                int right = nums.length - 1;

                while (left < right) {
                    int sum = nums[i] + nums[j] + nums[left] + nums[right];
                    if (sum < target) {
                        left++;
                    } else if (sum > target) {
                        right--;
                    } 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--;
                    }
                }
            }
        }
        return res;
    }
}