leetcode 15. 3Sum

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]]

解：这一题跟twosum有点类似，但是又有所不同。这一题先第一次遍历，得到所有需要得到的b+c的数，就需要在剩下的数组里面进行twosum，由于需要得到所有的情况，在进行twosum时如果按照之前的hash方法也可以，需要排除一下重复的情况。这里就是参考他人的ac，发现将数组排序之后再进行查找也很方便。

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