[Leetcode] 4Sum

 

Given an arraySOfNIntegers, are there elementsA,B,C, AndDInSSuch thatA+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≤B≤C≤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)
 

 

Two elements are enumerated, and the next two elements adopt the 2sum double-pointer approach.

 

1st (9 tries)

Class solution {public: vector <int> foursum (vector <int> & num, int target) {// cubic algorithm with a time complexity of N, first locate the first two, and then the last two ideas that use 2 sum // deduplication may not be very good. Vector <vector <int> re; vector <int> tmpnum; if (Num. size () <4) return re; sort (Num. begin (), num. end (); For (INT I = 0; I <num. size ()-3; I ++) {if (I> 0 & num [I] = num [I-1]) continue; for (Int J = I + 1; j <num. size ()-2; j ++) {If (j> I + 1 & num [J] = num [J-1]) continue; tmpnum. clear (); tmpnum. push_back (Num [I]); tmpnum. push_back (Num [J]); int tmptarget = target-num [I]-num [J]; int start = J + 1; int end = num. size ()-1; while (start <End) {If (Num [start] + num [end] = tmptarget) {If (Start> J + 1 & End <num. size ()-1 & num [start] = num [start-1] & num [end] = num [end + 1]) {start ++; end --; continue;} tmpnum. push_back (Num [start]); tmpnum. push_back (Num [end]); Re. push_back (tmpnum); tmpnum. pop_back (); tmpnum. pop_back (); Start ++; end --;} else if (Num [start] + num [end]> tmptarget) {end --;} else {start ++ ;}}} return re ;}};

2nd (3 tries)

Class solution {public: vector <int> ans; vector <int> res; vector <int> foursum (vector <int> & num, int target) {int n = num. size (); sort (Num. begin (), num. end (); int _ 1, _ 2, _ 3, _ 4; for (_ 1 = 0; _ 1 <n-3; _ 1 + +) {If (_ 1> 0 & num [_ 1] = num [_ 1-1]) continue; For (_ 2 = _ 1 + 1; _ 2 <N-2; _ 2 + +) {If (_ 2> _ 1 + 1 & num [_ 2] = num [_ 2-1]) continue; _ 3 = _ 2 + 1; _ 4 = n-1; while (_ 3 <_ 4) {If (_ 3> _ 2 + 1 & _ 4 <n-1 & num [_ 3] = num [_ 3-1] & num [_ 4] = = num [_ 4 + 1]) {_ 3 ++; _ 4 --; continue ;} if (Num [_ 1] + num [_ 2] + num [_ 3] + num [_ 4] = target) {res. push_back (Num [_ 1]); Res. push_back (Num [_ 2]); Res. push_back (Num [_ 3]); Res. push_back (Num [_ 4]); ans. push_back (RES); Res. clear (); _ 3 ++; _ 4 --;} else if (Num [_ 1] + num [_ 2] + num [_ 3] + num [_ 4]> Target) {_ 4 --;} else {_ 3 ++ ;}}} return ans ;}};