18 四数之和 Medium

Problem: 18. 四数之和

解题方法

遍历第一个和第二个元素，同时防止其与上一个元素相同 双指针遍历第三个和第四个元素，减少时间复杂度

复杂度

• 时间复杂度: $O(n^3)$

• 空间复杂度: $O(1)$

Code

class Solution:
    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
        n = len(nums)
        nums.sort()
        res = []
        for i in range(n - 3):
            if i > 0 and nums[i] == nums[i - 1]:
                continue
            for j in range(i + 1, n - 2):
                if j > i + 1 and nums[j] == nums[j - 1]:
                    continue
                left = j + 1
                right = n - 1
                rest = target - nums[i] - nums[j]
                while left < right:
                    if rest == nums[left] + nums[right]:
                        res.append([nums[i], nums[j], nums[left], nums[right]])
                        left += 1
                        while left < n and nums[left] == nums[left - 1]:
                            left += 1
                        right -= 1
                        while right > j + 1 and nums[right] == nums[right + 1]:
                            right -= 1          
                    elif rest > nums[left] + nums[right]:
                        left += 1
                    else:
                        right -= 1
        return res
                    

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