LeetCode_3 sum

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

Note:

Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ? b ? c)
The solution set must not contain duplicate triplets.
    For example, given array S = {-    - -},

    A solution set is:
    (-, , )
    (-, -, )

双指针的思想。使用DFS球排列组合时去重复的思想去重复

class Solution {
public:
    vector<vector<int> > threeSum(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int>> res;
        sort(num.begin(), num.end());
        int len = num.size();
        if(len < ) return res;

        for(int i = ; i <= len - ; ++i)
        {
            while(i>  && i <= len - && num[i] == num[i-]) ++i;
            if(i > len -) break;
            int low = i + ;
            int high = len -;
            while(low < high){

                int sum = num[i] + num[low] + num[high];
                if(sum == ){
                    vector<int> tp;
                    tp.push_back(num[i]);
                    tp.push_back(num[low]);
                    tp.push_back(num[high]);
                    res.push_back(tp);
                    ++low;
                    while(low < high && num[low] == num[low-])++low;
                    --high;
                    while(low <high && num[high] == num[high+]) --high;
                }else if(sum < ){
                    ++low;
                    while(low < high && num[low] == num[low-])++low;
                }else{
                    --high;
                    while(low < high && num[high] == num[high+]) --high;
                }
            }
        }

        return res;
    }
};