Difficulty: Medium
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]
]
Language: Java
class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
if (nums.length < 4) {
return Collections.emptyList();
}
Arrays.sort(nums);
Set<List<Integer>> result = new HashSet<>(); // 一开始写的是list，不过这个去重真的是很麻烦，还是使用set来去重了
for (int i = 0; i < nums.length - 1; i++) {
if (i > 0 && nums[i] == nums[i - 1]) {
continue;
}
for (int j = i + 1; j < nums.length; j++) {
int targetSum = target - (nums[i] + nums[j]);
if (targetSum < nums[j] && nums[j] > 0) { // 因为已经排序了，所以后面的任意两个数和必然大于 targetSum
break;
}
for (int x = j + 1, y = nums.length - 1; x < y; ) {
if (nums[x] + nums[y] > targetSum) {
y--;
} else if (nums[x] + nums[y] < targetSum) {
x++;
} else {
result.add(Arrays.asList(nums[i], nums[j], nums[x], nums[y]));
x++;
y--;
while (x < y && x >= 1 && nums[x] == nums[x - 1]) {
x++;
}
while (x < y && y + 1 < nums.length && nums[y] == nums[y + 1]) {
y--;
}
}
}
}
}
return new ArrayList<>(result);
}
}