LeetCode 18. 四数之和

短命女 2022-10-30 15:25 116阅读 0赞

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODE5OTc3MA_size_16_color_FFFFFF_t_70]

//两个外层循环固定a和b，适当的进行提前判断，里层两个指针往里逼近
    class Solution {
        
        public List<List<Integer>> fourSum(int[] nums, int target) {
        
            List<List<Integer>> quadruplets = new ArrayList<List<Integer>>();
            if (nums == null || nums.length < 4) {
        
                return quadruplets;
            }
            Arrays.sort(nums);
            int length = nums.length;
            for (int i = 0; i < length - 3; i++) {
        
                if (i > 0 && nums[i] == nums[i - 1]) {
        
                    continue;
                }
                if (nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3] > target) {
        
                    break;
                }
                if (nums[i] + nums[length - 3] + nums[length - 2] + nums[length - 1] < target) {
        
                    continue;
                }
                for (int j = i + 1; j < length - 2; j++) {
        
                    if (j > i + 1 && nums[j] == nums[j - 1]) {
        
                        continue;
                    }
                    if (nums[i] + nums[j] + nums[j + 1] + nums[j + 2] > target) {
        
                        break;
                    }
                    if (nums[i] + nums[j] + nums[length - 2] + nums[length - 1] < target) {
        
                        continue;
                    }
                    int left = j + 1, right = length - 1;
                    while (left < right) {
        
                        int sum = nums[i] + nums[j] + nums[left] + nums[right];
                        if (sum == target) {
        
                            quadruplets.add(Arrays.asList(nums[i], nums[j], nums[left], nums[right]));
                            while (left < right && nums[left] == nums[left + 1]) {
        
                                left++;
                            }
                            left++;
                            while (left < right && nums[right] == nums[right - 1]) {
        
                                right--;
                            }
                            right--;
                        } else if (sum < target) {
        
                            left++;
                        } else {
        
                            right--;
                        }
                    }
                }
            }
            return quadruplets;
        }
    }

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