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3-sum.cpp
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3-sum.cpp
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// Time: O(n^2)
// Space: O(1)
class Solution {
public:
/**
* @param numbers : Give an array numbers of n integer
* @return : Find all unique triplets in the array which gives the sum of zero.
*/
vector<vector<int>> threeSum(vector<int> &nums) {
vector<vector<int>> ans;
const int target = 0;
// Make nums in increasing order. Time: O(nlogn)
sort(nums.begin(), nums.end());
for (int i = 0; i < nums.size() - 2; ++i) {
// Skip duplicated.
if (i > 0 && nums[i] == nums[i - 1]) {
continue;
}
int j = i + 1;
int k = nums.size() - 1;
while (j < k) { // Time: O(n) for each i.
if (j - 1 > i && nums[j] == nums[j - 1]) { // Skip duplicated.
++j;
} else if (k + 1 < nums.size() && nums[k] == nums[k + 1]) { // Skip duplicated.
--k;
} else {
const auto sum = nums[i] + nums[j] + nums[k];
if (sum > target) { // Should decrease sum.
--k;
} else if (sum < target) { // Should increase sum.
++j;
} else {
ans.emplace_back(vector<int>{nums[i], nums[j], nums[k]});
++j;
--k;
}
}
}
}
return ans;
}
};