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)

解题思路：

　　4个指针， 前两个指针双循环遍历数组，第三个指针放在第二个指针之后，第四个指针放在末尾，求和和target比较，如果 sum > target 最后一个指针--， 如果sum < target 第三个指针++; 相等先判断是否是重复的（hashset）,如果不是添加到result中去；

public class Solution {
    public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
        Arrays.sort(num);
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        HashSet<ArrayList<Integer>> set = new HashSet<ArrayList<Integer>>();
        int len = num.length;
        for(int i = 0; i< len; i++){
            for(int j = i+1; j < len; j++){
                int k = j+1;
                int l = len-1;
                
                while(k < l){
                  int sum = num[i]+num[j]+num[k]+num[l];
                        
                        if(sum < target){
                            k++;
                        }else if(sum > target){
                            l--;
                        }else {
                            ArrayList<Integer> output = new ArrayList<Integer>();
                            output.add(num[i]);
                            output.add(num[j]);
                            output.add(num[k]);
                            output.add(num[l]);
                            if(! set.contains(output)){
                                set.add(output);
                                result.add(output);
                            }
                            k++;
                            l--;
                        }      
                }   
            }
        }
        
        return result;
        
    }
}