18. 4Sum
Given an array nums of n integers, return an array of all the unique quadruplets [nums[a], nums[b], nums[c], nums[d]] such that:
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0 <= a, b, c, d < n
a, b, c, and d are distinct.
nums[a] + nums[b] + nums[c] + nums[d] == target
You may return the answer in any order.
Example 1:
Input: nums = [1,0,-1,0,-2,2], target = 0
Output: [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]
Example 2:
Input: nums = [2,2,2,2,2], target = 8
Output: [[2,2,2,2]]
C++ Solution
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class Solution {
public:
struct VectorHash {
size_t operator()(const std::vector<int>& v) const {
std::hash<int> hasher;
size_t seed = 0;
for (int i : v) {
seed ^= hasher(i) + 0x9e3779b9 + (seed<<6) + (seed>>2);
}
return seed;
}
};
using MySet = std::unordered_set<std::vector<int>, VectorHash>;
vector<vector<int>> fourSum(vector<int>& nums, int target) {
vector<vector<int>> ret;
MySet my_set;
int len = nums.size();
sort(nums.begin(), nums.end());
for (int i = 0; i < len;i++) {
long long second_target = target - nums[i];
for (int j = i + 1;j < len;j++) {
long long third_target = second_target - nums[j];
int left = j + 1;
int right = len - 1;
while (left < right) {
if (third_target == nums[left] + nums[right]) {
auto tmp = vector<int>{nums[i],nums[j],nums[left],nums[right]};
if (my_set.count(tmp) == 0){
ret.push_back(tmp);
my_set.insert(tmp);
}
left++;
} else if (third_target > nums[left] + nums[right]) {
left++;
} else {
right--;
}
}
}
}
return ret;
}
};
