思路:
一般会用到hash,这道题参考花花酱的。
class Solution {
public:
vector<vector<int>> fourSum(vector<int>& nums, int target) {
sort(nums.begin(), nums.end());
if (target > 0 && target > 4 * nums.back()) return {};
if (target < 0 && target < 4 * nums.front()) return {};
unordered_map<int, int> index;
for (int i = 0; i < nums.size(); ++i) {
index[nums[i]] = i;
}
set<vector<int>> h;
int h = nums.size();
for (int i = 0; i < n; ++i) {
for (int j = i + 1; j < n; ++j) {
for (int k = j + 1; k < n; ++k) {
int t = target - nums[i] - nums[j] - nums[k];
if (t < nums[k]) break;
auto it = index.find(t);
if (it == index.end() || it->second <= k) continue;
h.insert({nums[i], nums[j], nums[k], t});
}
}
}
return vector<vector<int>> (begin(h), end(h));
}
};
参考:
https://zxi.mytechroad.com/blog/algorithms/binary-search/leetcode-18-4sum/