LeetCode_18 4Sum
Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b+ c + d = target? Find all unique quadruplets in the array which gives the sum of target.
Note:
The solution set must not contain duplicate quadruplets.
Example:
Given array nums = [1, 0, -1, 0, -2, 2], and target = 0.
A solution set is:
[
[-1, 0, 0, 1],
[-2, -1, 1, 2],
[-2, 0, 0, 2]
]
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 - 3; i++) {
if (i != 0 && nums[i] == nums[i - 1])
continue;
for (int j = i + 1; j < nums.length - 2; j++) {
if (j > i + 1 && nums[j] == nums[j - 1])
continue;
int k = j + 1;
int l = nums.length - 1;
while (k < l) {
int sum = nums[i] + nums[j] + nums[k] + nums[l];
if (sum == target) {
List<Integer> list = new ArrayList<>();
list.add(nums[i]);
list.add(nums[j]);
list.add(nums[k]);
list.add(nums[l]);
res.add(list);
k++;
l--;
// 去重复
while (k < l && nums[k] == nums[k - 1]) {
k++;
}
while (k < l && nums[l] == nums[l + 1]) {
l--;
}
} else if (sum < target) {
k++;
} else {
l--;
}
}
}
}
return res;
}
public List<List<Integer>> fourSum3(int[] num, int target) {
ArrayList<List<Integer>> ans = new ArrayList<>();
if (num.length < 4)
return ans;
Arrays.sort(num);
for (int i = 0; i < num.length - 3; i++) {
if (num[i] + num[i + 1] + num[i + 2] + num[i + 3] > target)
break; // first candidate too large, search finished
if (num[i] + num[num.length - 1] + num[num.length - 2] + num[num.length - 3] < target)
continue; // first candidate too small
if (i > 0 && num[i] == num[i - 1])
continue; // prevents duplicate result in ans list
for (int j = i + 1; j < num.length - 2; j++) {
if (num[i] + num[j] + num[j + 1] + num[j + 2] > target)
break; // second candidate too large
if (num[i] + num[j] + num[num.length - 1] + num[num.length - 2] < target)
continue; // second candidate too small
if (j > i + 1 && num[j] == num[j - 1])
continue; // prevents duplicate results in ans list
int low = j + 1, high = num.length - 1;
while (low < high) {
int sum = num[i] + num[j] + num[low] + num[high];
if (sum == target) {
ans.add(Arrays.asList(num[i], num[j], num[low], num[high]));
while (low < high && num[low] == num[low + 1])
low++; // skipping over duplicate on low
while (low < high && num[high] == num[high - 1])
high--; // skipping over duplicate on high
low++;
high--;
}
// move window
else if (sum < target)
low++;
else
high--;
}
}
}
return ans;
}
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