LeetCode----- 18.4Sum

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: 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]
]

给定数组S的n个整数，元素a,b,c,d，这样a + b + c + d =目标吗?在数组中找到所有唯一的四胞胎，给出了目标的总和。

解题思路：求4Sum，跟求3sum方法类似，都是一步一步的转换成求2sum的方法。

代码如下：

public static List> fourSum(int[] nums, int target) {
    	List> list = new ArrayList>();
    	if(nums.length <4) {
    		return list;
    	}
    	Arrays.sort(nums);
    	for (int i = 0; i < nums.length-3; i++) {
    		if(i>0 && nums[i] == nums[i-1]) {
    			continue;
    		}
    		for (int j = i+1; j < nums.length-2; j++) {
				int start = j+1;
				int end = nums.length-1;
				while(start result =Arrays.asList(nums[i],nums[j],nums[start],nums[end]);
						if(!list.contains(result)) {
							list.add(result);
						}						
						start++;
						end--;
					}else if(sum < target) {
						start++;
					}else {
						end--;
					}
				}
			}
		}
    	return list;
    }