[LeetCode]4Sum

4Sum

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Total Accepted: 50999 Total
Submissions: 230927 Difficulty: Medium

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)

a

这个题目跟3Sum的题目是一样的，只不过多了一个循环，去重也多了一步，对于第二个数也不能跟它之前的数重复。在找到一个解之后也要继续去重。

public class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
Arrays.sort(nums);
ArrayList<List<Integer>>result = new ArrayList<List<Integer>>();
if(nums.length<4)return result;
for(int i = 0;i<=nums.length-4;i++){
if(i!=0&&nums[i]==nums[i-1]){
continue;
}
for(int j =i+1;j<=nums.length-3;j++){
if(j!=i+1&&nums[j]==nums[j-1]){
continue;
}
int p = j+1;int q = nums.length-1;
while(p<q){
int sumTemp = nums[i]+nums[j]+nums[p]+nums[q];
if(sumTemp==target){
ArrayList<Integer>sumArrayList = new ArrayList<Integer>();
sumArrayList.add(nums[i]);sumArrayList.add(nums[j]);
sumArrayList.add(nums[p]);sumArrayList.add(nums[q]);
result.add(sumArrayList);
p++;
while(p<q&&nums[p]==nums[p-1]){
p++;
}
q--;
while(p<q&&nums[q]==nums[q+1]){
q--;
}
}
else{
if(sumTemp>target){
q--;
}
else{
p++;
}
}
}

}
}
return result;
}
}