LeetCode_OJ【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:

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)

此题思路和
LeetCode_OJ【15】3Sum 以及
LeetCode_OJ【16】3Sum Closest 思路类似，这里不再赘述，实现如下：

public class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
Arrays.sort(nums);
List<List<Integer>> list = new ArrayList<List<Integer>>();
for(int i = 0 ; i < nums.length - 3 ; i++){
if(i != 0 && nums[i-1] == nums[i])
continue;
for(int j = i + 1; j < nums.length -2 ; j++){
if(j != i+1 && nums[j-1] == nums[j])
continue;
for(int left = j + 1,right = nums.length -1; left < right ;){
if(nums[i] + nums[j] + nums[left] + nums[right] == target){
if(left != j + 1 && nums[left-1] == nums[left]){
left ++;
continue;
}
List<Integer> tmp = new ArrayList<Integer>();
tmp.add(nums[i]);
tmp.add(nums[j]);
tmp.add(nums[left]);
tmp.add(nums[right]);
list.add(tmp);
left ++;
}
else if(nums[i] + nums[j] + nums[left] + nums[right] > target){
right --;
}
else {
left ++ ;
}
}
}
}
return list;
}
}