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;              } };