leetcode 44: 4Sum

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

Note:

• Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a â‰¤ b â‰¤ c â‰¤ d)
• The solution set must not contain duplicate quadruplets.

    For example, given array S = {1 0 -1 0 -2 2}, and target = 0.

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

 

 

class Solution {
 public:
     vector<vector<int> > fourSum(vector<int> &num, int target) {
         // Start typing your C/C++ solution below
         // DO NOT write int main() function
         
         vector<vector<int> > rel;
         int sz = num.size();
         if(sz<4) return rel;
         sort( num.begin(), num.end() );
         set< vector<int> > mySet;
         
         
         for( int i=0; i<sz-3; i++) {
             for( int j=i+1; j<sz-2; j++) {
                 int l = j + 1;
                 int h = sz-1;
                 while( l < h) {
                     int temp = num[i] + num[j] + num[l] + num[h];
                     if( temp == target){
                         vector<int> vec;
                         vec.push_back( num[i]);
                         vec.push_back( num[j]);
                         vec.push_back( num[l]);
                         vec.push_back( num[h]);
                         if( mySet.find( vec ) == mySet.end() ) {
                            rel.push_back( vec );
                            mySet.insert( vec );
                         }
                         ++l;
                         --h;
                     } else if( temp < target) {
                         ++l;
                     } else {
                         --h;
                     }
                 }
             }
         }
         return rel;
         
     }
 };