15. 3Sum

two pointers O(n²)

class Solution:
    def threeSum(self, nums: List[int]) -> List[List[int]]:
        nums.sort()
        n = len(nums)
        res = []
        for i in range(n):
            if i > 0 and nums[i - 1] == nums[i]: continue
            t = -nums[i]
            l, r = i + 1, n - 1
            while l < r:
                s = nums[l] + nums[r]
                if s == t:
                    res.append([nums[i], nums[l], nums[r]])
                    l += 1
                    while l < r and nums[l - 1] == nums[l]: l += 1
                    r -= 1
                    while l < r and nums[r] == nums[r + 1]: r -= 1
                elif s < t:
                    l += 1
                    while l < r and nums[l - 1] == nums[l]: l += 1
                else:
                    r -= 1
                    while l < r and nums[r] == nums[r + 1]: r -= 1
        return res

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        vector<vector<int>> res;
        sort(nums.begin(), nums.end());
        int n = nums.size();
        for (int i = 0; i < n; ++i) {
            if (i > 0 && nums[i] == nums[i - 1]) continue;
            long t = -(long)nums[i];
            int l = i + 1, r = n - 1;
            while (l < r) {
                long s = (long)nums[l] + nums[r];
                if (s == t) {
                    res.push_back({nums[i], nums[l], nums[r]});
                    do { ++l; } while (l < r && nums[l] == nums[l - 1]);
                    do { --r; } while (l < r && nums[r] == nums[r + 1]);
                } else if (s < t) {
                    do { ++l; } while (l < r && nums[l] == nums[l - 1]);
                } else {
                    do { --r; } while (l < r && nums[r] == nums[r + 1]);
                }
            }
        }
        return res;
    }
};