"Leetcode with Java" 4Sum (on^2)

I saw a few popular high blog, they do not do this algorithm O (n^2), so I put the solution in advance to share, for your reference (the topic of the previous skip will be made up in the near future).

"title":

Given an array S of n integers, is 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 is 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)

"On the Code"
Import Java.util.arraylist;import java.util.collections;import java.util.enumeration;import java.util.Hashtable; Import java.util.list;import java.util.vector;/* * Reference: No reference to others algorithm * O (n^2) * idea: first to plug each logarithm into a hashtable (with this logarithm and key, Value as a set containing all and key pairs). * Then traverse the Hashtable, for each set of pairs of keys, the set of get and number pairs for target-key in that Hashtable, * if get to is empty, then continue, if get to is not empty, Then set these two pairs and then an ordered 4-tuple, and see if the previous * 4-tuple has already appeared (using another hastable<string,anytype>:reappear to quickly determine the reappear's * Key is a concatenation of 4 integers with an ordered 4-tuple). If the 4-tuple is present, it is added to the result set. * */public class Solution1 {public list<list<integer>> foursum (int[] num, int target) {List<list<integ er>> list = new arraylist<list<integer>> (); if (num = = NULL | | Num.length < 4) return List;int len = nu M.length; hashtable<integer,pairrecoder> prs = new hashtable<integer,pairrecoder> (Len*len); for (int i = 0; i < len- 1; i++) {for (int j = i + 1; j < Len; J + +) {int sum = Num[i]+num[j]; Pairrecoder PR = prs.get (num[i]+num[j]); if (pr = = null) prs.put (Sum, new Pairrecoder (SUM,I,J)), Elsepr.pairs.add (New Pair (I,J));}} List<four> fours = new arraylist<four> (); hashtable<string,boolean> reappear = new hashtable<string,boolean> (); For (enumeration<pairrecoder> e=prs.elements (); e.hasmoreelements ();) {Pairrecoder PR1 = (pairrecoder) e.nextelement (); Pairrecoder PR2 = Prs.get (TARGET-PR1.V), if (PR2 = = null) continue;for (int i=0;i<pr1.pairs.size (); i++) for (int j=0;j <pr2.pairs.size (); j + +) {Pair P1 = pr1.pairs.elementAt (i); Pair P2 = pr2.pairs.elementAt (j); if (p1.v1!=p2.v1 &&p1.v1!=p2.v2 && p1.v2!=p2.v1 && p1.v2!= P2.V2) {Four of = new Orderedfour (Num[p1.v1],num[p1.v2],num[p2.v1],num[p2.v2]); StringBuffer sb = new StringBuffer (), Sb.append (of.num.get (0)), Sb.append (Of.num.get (1)), Sb.append (Of.num.get (2)); Sb.append (Of.num.get (3)), if (!reappear.containskey (sb.tostring ())) {List.add (of.num); System.out.println ("NEW:" +SB); Reappear.put (Sb.tostring (), True);}}} return list;} Class Pair {int V1;int v2; Pair (intv1, int v2) {this.v1 = V1;this.v2 = v2;}} Class Pairrecoder{int V; vector<pair> pairs = new vector<pair> (); Pairrecoder (int v,int i,int j) {this.v = V;this.pairs.add (new Pair (I,J));}} Class four{list<integer> num; Four (int v1,int v2,int v3,int v4) {this.num = new arraylist<integer> (); Num.add (v1); Num.add (v2); Num.add (v3); Num.add (v4);}} Class Orderedfour extends Four{orderedfour (int v1,int v2,int V3,int v4) {Super (V1, V2, v3, v4); Collections.sort (This.num);}}}




"Leetcode with Java" 4Sum (on^2)