Leetcode: 4Sum

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Problem:

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 ≤ 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)

Code

public class Solution {    public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {        ArrayList<ArrayList<Integer>>  result = new ArrayList<ArrayList<Integer>>();        Arrays.sort(num);        for (int i = 0; i < num.length - 3  ; i++) {            if (i != 0 && num[i] == num[i - 1]) continue;            for (int j = i + 1; j < num.length - 2; j++) {                if ( j != i + 1 && num[j] == num[j - 1]) continue;                int key = target - num[i] - num[j];                int start = j + 1;                int end = num.length - 1;                while (start < end) {                    if (num[start] + num[end] == key) {                        ArrayList<Integer> list = new ArrayList<Integer>();                        list.add(num[i]);                        list.add(num[j]);                        list.add(num[start]);                        list.add(num[end]);                        result.add(list);                        start++;                        end--;                        while (start < end && num[start] == num[start - 1]) start++;                        while(start < end && num[end] == num[end + 1]) end--;                     } else if (num[start] + num[end] < key) {                        start++;                    } else end--;                }            }                      }                return result;    }}

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