LeetCode:3Sum

problem:

 

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)
Solution:  先排序 然后左右夹逼 时间复杂度：O(n2)

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
         vector<vector<int>> result;
         
         if(nums.size()<3) return result;
         //先排序 然后再左右夹逼
         sort(nums.begin(),nums.end());
         
         const int target=0;
         auto last=nums.end();
         for(auto a=nums.begin();a<prev(last,2);a++)
         {
                  auto b = next(a);
                  auto c= prev(last);
                  
                  while(b<c)
                  {
                    if(*a+*b+*c<target)
                       b++;
                    else if(*a+*b+*c>target)
                     --c;
                     else{
                          
                          result.push_back({*a,*b,*c});
                          ++b;
                          --c;
                          }
                   }
                  
              }
              return result;
              }
};