Description
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] ]
思路
题意：给出一串整数值序列，输出a + b + c + d = target的方案
题解：此题与3Sum类似，区别在于这是求取四个数的和等于目标值的方案，因此将其降维为求取3Sum问题。
class Solution {
//61ms
public List<List<Integer>> fourSum(int[] nums, int target) {
Arrays.sort(nums);
List<List<Integer>>res = new ArrayList<>();
int len = nums.length;
for (int i = 0;i < len - 3;i++){
threeSum(nums,nums[i],target - nums[i],i + 1,len,res);
while (i + 1 < len && nums[i + 1] == nums[i]){
i++;
}
}
return res;
}
public void threeSum(int[] nums,int val,int target,int lo,int ro,List<List<Integer>>res) {
for (int i = lo;i < ro - 2;i++){
int left = i + 1,right = ro - 1;
int sum = target - nums[i];
while (left < right){
if (nums[left] + nums[right] < sum){
left++;
} else if (nums[left] + nums[right] > sum){
right--;
} else{
res.add(Arrays.asList(val,nums[i],nums[left],nums[right]));
while (++left < right && nums[left] == nums[left - 1]){}
while (--right > left && nums[right] == nums[right + 1]){}
}
}
while (i + 1 < ro - 2 && nums[i + 1] == nums[i]){
i++;
}
}
}
}