Title
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]
]
注意不能冗余，所以可以判断每个数是否冗余，但其实判断3个就够了，三个不冗余，第四个即使duplicate也不等于target了
Solution
//cpp:
class Solution {
public:
vector<vector<int>> fourSum(vector<int>& nums, int target) {
vector<vector<int>> res;
sort(nums.begin(), nums.end());
for (int i = 0; i < nums.size(); i++) {
if (i > 0 && nums[i] == nums[i - 1]) continue;
for (int j = i + 1; j < nums.size(); j++) {
if (j > i + 1 && nums[j] == nums[j - 1]) continue;
int lo = j + 1, hi = nums.size() - 1;
while (lo < hi) {
int sum = nums[i] + nums[j] + nums[lo] + nums[hi];
if (sum == target) {
res.push_back({ nums[i],nums[j],nums[lo],nums[hi] });
while (lo<hi&&nums[lo] == nums[lo + 1]) lo++;
//while (lo<hi&&nums[hi] == nums[hi - 1]) hi--;
lo++;
hi--;
}
else if (sum > target) {
hi--;
}
else {
lo++;
}
}
}
}
return res;
}
};