3Sum.cpp

#include <iostream>
#include <vector>

class Solution {
  public:
    std::vector<std::vector<int>> threeSum(std::vector<int> &nums) {
        std::vector<std::vector<int>> res;
        int n = nums.size();
        if (n < 3) {
            return res;
        }
        std::sort(nums.begin(), nums.end());
        for (int i = 0; i < n - 2; ++i) {
            if (nums[i] > 0) {
                break;
            }
            if (i > 0 && nums[i] == nums[i - 1]) {
                continue;
            }
            int j = i + 1;
            int k = n - 1;
            while (j < k) {
                int sum = nums[i] + nums[j] + nums[k];
                if (sum == 0) {
                    res.push_back({nums[i], nums[j], nums[k]});
                    while (j < k && nums[j] == nums[j + 1]) {
                        j++;
                    }
                    j++;
                    while (j < k && nums[k] == nums[k - 1]) {
                        k--;
                    }
                    k--;
                } else if (sum < 0) {
                    j++;
                } else {
                    k--;
                }
            }
        }
        return res;
    }
};