15. Three Sum

The description of the problem is : https://leetcode.com/problems/3sum/#/description

Generally, The time complexity is O(n^2). Use for loop and squeeze rule to solve this problem.

For boundary condition: consider when the number in candidates is less than 3, then there is no solution.

The solution:

class Solution {
public:
vector<vector<int>> threeSum(vector<int>& nums) {
if (nums.size() < 3) return{};
vector<vector<int>> result;
sort(nums.begin(), nums.end());
for (int i = 0; i < nums.size() – 2; i++)
{
if (nums[i] > 0)
break;
if (i > 0 && nums[i] == nums[i – 1])
continue;
int left = i + 1;
int right = nums.size() – 1;

while (left < right)
{
if (nums[right] < 0)
break;
int tmp = nums[i] + nums[left] + nums[right];
if (tmp == 0)
{
result.push_back({ nums[i], nums[left], nums[right] });
while (left < right && nums[left] == nums[left + 1])
left++;
while (left < right && nums[right] == nums[right – 1])
right–;
++left;
–right;
}
else if (tmp < 0)
left++;
else
right–;
}
}
return result;
}
};

Tips:

How to remove the duplicate elements in vector?

result.erase(std::unique(result.begin(), result.end()), result.end());