LeetCode_3 sum

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:

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 = {-1 0 1 2 -1 -4},

    A solution set is:
    (-1, 0, 1)
    (-1, -1, 2)

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 < 3) return res;
        
        for(int i = 0; i <= len -3 ; ++i)
        {
            while(i> 0 && i <= len -3 && num[i] == num[i-1]) ++i;
            if(i > len -3) break;
            int low = i + 1;
            int high = len -1;
            while(low < high){
            
                int sum = num[i] + num[low] + num[high];
                if(sum == 0){
                    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-1])++low;
                    --high;
                    while(low <high && num[high] == num[high+1]) --high;
                }else if(sum < 0){
                    ++low;
                    while(low < high && num[low] == num[low-1])++low;
                }else{
                    --high;
                    while(low < high && num[high] == num[high+1]) --high;
                }
            }
        }
        
        return res;
    }
};