Leetcode: 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 ≤ 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)

思路：和3sum思想一致，排序后，遍历最小值，并往右相加。

public class Solution {    public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();        if (num == null || num.length < 4) {            return res;        }                Arrays.sort(num);        for (int first = 0; first < num.length - 3; first++) {            if (first != 0 && num[first] == num[first - 1]) {                continue;            }            for (int second = first + 1; second < num.length - 2; second++) {                if (second != first + 1 && num[second] == num[second - 1]) {                    continue;                }                int third = second + 1;                int fourth = num.length - 1;                while (third < fourth) {                    int sum = num[first] + num[second] + num[third] + num[fourth];                    if (sum == target) {                        ArrayList<Integer> item = new ArrayList<Integer>();                        item.add(num[first]);                        item.add(num[second]);                        item.add(num[third]);                        item.add(num[fourth]);                        res.add(item);                        third++;                        fourth--;                        while (third < fourth && num[third] == num[third - 1]) {                            third++;                        }                        while (third < fourth && num[fourth] == num[fourth + 1]) {                            fourth--;                        }                    } else if (sum > target) {                        fourth--;                    } else {                        third++;                    }                }            }        }                return res;    }}