4sum

packagecom.ytx.hash;

importjava.util.ArrayList;

importjava.util.Arrays;

/** 题目： 4sum

 *

 *     描述： 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 uniquequadruplets

 *                  in the array which gives the sum of target.

                    Note:

                    Elements in aquadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)

                    The solution set must not contain duplicatequadruplets.

                    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)

 * 

 *@authoryuantian xin

 *

 */

publicclass Four_Sum {

       

       ArrayList< ArrayList<Integer> >res;

       publicArrayList<ArrayList<Integer>> fourSum(int[]num,int target) {

             intlen = num.length;

             res= new ArrayList<ArrayList<Integer>>();

             if(num== null || len == 0) return res;

             Arrays.sort(num);

             inti = 0, j = 0;

             for(i= 0; i < len - 3; i++) {

                    if(i != 0 && num[i] ==num[i- 1]) {

                           continue;

                    }

                    

                    for(j= i + 1; j < len - 2; j++) {

                           if(j != i + 1 && num[j] ==num[j- 1]) {

                                 continue;

                           }

                           

                           TwoSum(num,i,j,j+1,len - 1, target);

                    }

             }

             

             returnres;

       }

       

       publicvoid TwoSum(int[]num,int i, int j,int begin, int end,int target) {

             while(begin< end) {

                    

                    intsum = num[i] +num[j] +num[begin] +num[end];

                    

                    if(sum< target) {

                           begin++;

                    }else if(sum> target) {

                           end--;

                    }else {

                           ArrayList<Integer>temp = newArrayList<Integer>();

                           temp.add(num[i]);

                           temp.add(num[j]);

                           temp.add(num[begin]);

                           temp.add(num[end]);

                           res.add(temp);

                           begin++;

                           end--;

                           while(begin< end && num[begin] ==num[begin- 1]) {

                                 begin++;

                           }

                           while(begin< end && num[end] ==num[end+ 1]) {

                                 end--;

                           }

                    }

             }

       }

       publicstatic void main(String[]args) {

             intnum[] = {1,0,-1,0,-2,2};

             ArrayList<ArrayList<Integer>>lists = newArrayList<>();

             lists= new Four_Sum().fourSum(num, 0);

             for(ArrayList<Integer>arrayi : lists) {

                    System.out.print("[");

                    for(Integeri : arrayi) {

                           System.out.print(i+ " ");

                    }

                    System.out.println("]");

             }

             

       }

}