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)

2013-07-04

只能通过小数据，使用的方法和3Sum类似，使用4个指针。

public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        int len = num.length;
        if(len < 4)
            return result;
        Arrays.sort(num);
        for(int i = 0; i < len - 3; i ++){
            for(int j = i + 1; j < len - 2; j++){
                int m = j + 1;
                int n = len -1;
                while(m < n){
                    int sum = num[m] + num[n];
                    if(sum == target - num[i] - num[j]){
                        ArrayList<Integer> list = new ArrayList<Integer>();
                        list.add(num[i]);
                        list.add(num[j]);
                        list.add(num[m]);
                        list.add(num[n]);
                        result.add(list);
                        m ++;
                        n --;
                        while(m < n && num[m - 1] == num[m]) m++;
                        while(m < n && num[n] == num[n + 1]) n--;
                    } else if(sum < target - num[i] - num[j]){
                        m ++;
                    } else {
                        n --;
                    }
                    
                }
                while(j < len -3 && num[j] == num[j + 1]) j++;
            }
            while(i < len - 4 && num[i] == num[i + 1]) i++;
        }
        return result;
    }