力扣15，三数之和

这题可以双指针法，用哈希表有点复杂

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