Leetcode 15. Sum（二分或者暴力或者哈希都可以）

15. 3Sum

Medium

Given an array nums of n integers, are there elements a, b, c in nums 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.

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

题解： 三个数的和，思路就是求任意两个数的和x，然后O(n^2),再用O(1)的搜索搜索-x是否再数组内就可以了，O(1)搜索用哈希就行，或者用O(longn）的二分的方法也可以接收，我用的自带的find函数，最后也过了

 class Solution {
 public:
     vector<vector<int>> threeSum(vector<int>& nums) {
         multiset<int> left;
         multiset<int> right;
         set<vector<int>> ans;
         int fl = ;
         for(int i = ;i < nums.size(); i++){
             if(nums[i] == ) fl++;
             else if(nums[i]<) left.insert(nums[i]);
             else if(nums[i]>) right.insert(nums[i]);
         }
         set<int>:: iterator lit;
         set<int>:: iterator rit;
         for(lit = left.begin(); lit!=left.end(); lit++){
             rit = lit;
             rit++;
             for(; rit != left.end(); rit++){
                 int tt = (*lit)+(*rit);
                 if(right.find(-tt)!=right.end()){
                     vector<int> New;
                     New.push_back( (*lit));New.push_back( (*rit));New.push_back( -tt);
                     ans.insert(New);
                 }
             }
         }
         for(rit = right.begin(); rit!=right.end(); rit++){
             lit = rit;
             lit++;
             for(; lit != right.end(); lit++){
                 int tt = (*lit)+(*rit);
                 if(left.find(-tt)!=left.end()){
                     vector<int> New;
                     New.push_back( -tt);New.push_back( (*rit));New.push_back( (*lit));
                     ans.insert(New);
                 }
             }
         }
         if(fl>=){
             set<int>:: iterator it;
             for(it = left.begin(); it!=left.end(); it++){
                 if(right.find(-(*it))!=right.end()){
                     vector<int> New;
                     New.push_back((*it));New.push_back();New.push_back(-(*it));
                     ans.insert(New);
                 }
             }
         }
         if(fl>=){
             vector<int> New;
             New.push_back();New.push_back();New.push_back();
             ans.insert(New);
         }
         vector<vector<int>> finalans;
         set<vector<int>>:: iterator i;
         for(i = ans.begin(); i!=ans.end(); i++){
             vector<int> t = (*i);
             finalans.push_back(t);
         }
         return finalans;
     }
 };