三数之和

排序+双指针

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

时间复杂度：O(N^2)
空间复杂度：O(logN)