leet code. 15. 3Sum （暴力？？？）

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note: The solution set must not contain duplicate triplets.

For example, given array S = [-1, 0, 1, 2, -1, -4],A solution set is:[  [-1, 0, 1],  [-1, -1, 2]]

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排一下序，然后从第一个开始，用两个指针一个指向头一个指向尾，向中间进行遍历，如果遇上一个数字相同就跳到下一个。

vector<vector<int>> threeSum(vector<int>& nums) {    vector<vector<int>> ans;    sort(nums.begin(), nums.end());    for (int i = 0; i < nums.size(); ++i) {        int num = nums[i];        if (num == nums[i - 1] && i > 0)            continue;        int left = i + 1;        int right = nums.size() - 1;        while (left < right) {                //cout << left << "    " << right << endl;            if (num + nums[left] + nums[right] < 0) {                left++;                while (nums[left] == nums[left - 1] && left < right)                    left++;            } else if (num + nums[left] + nums[right] > 0) {                right--;                while (nums[right] == nums[right + 1] && left < right)                    right--;            } else {                ans.push_back({num, nums[left], nums[right]});                left++;                while (nums[left] == nums[left - 1] && left < right)                    left++;            }        }    }    return ans;}