4Sum(Medium)

Leetcode Source

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:

    1. Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
    2. 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)

使用HashSet来解决重复问题的代码如下：

public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
    HashSet<ArrayList<Integer>> hashSet = new HashSet<ArrayList<Integer>>();
    ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
    Arrays.sort(num);

    for (int i = 0; i <= num.length-4; i++) {
        for (int j = i + 1; j <= num.length-3; j++) {
            int low = j + 1;
            int high = num.length - 1;

            while (low < high) {
                int sum = num[i] + num[j] + num[low] + num[high];

                if (sum > target) {
                    high--;
                } else if (sum < target) {
                    low++;
                } else if (sum == target) {
                    ArrayList<Integer> temp = new ArrayList<Integer>();
                    temp.add(num[i]);
                    temp.add(num[j]);
                    temp.add(num[low]);
                    temp.add(num[high]);

                    if (!hashSet.contains(temp)) {
                        hashSet.add(temp);
                        result.add(temp);
                    }

                    low++;
                    high--;
                }
            }
        }
    }

    return result;
}

References

1 Summary for LeetCode 2Sum, 3Sum, 4Sum, K Sum