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]]

如果三个数加起来等于0，则符合条件。找出所有的符合条件的三个数的组合。这里的想法是先定下一个数，然后用left,right两端索引移动来搜索。要注意不能重复。

代码：

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