# 18.4Sum

## ้ข็ฎๅฐๅ

https://leetcode.com/problems/4sum/

https://www.lintcode.com/problem/4sum/description

## ้ข็ฎๆ่ฟฐ

``````Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums 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.

Example:
Given array nums = [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]
]``````

## ไปฃ็ 

### Approach #2 Two for-loops + Two points

``````public class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
List<List<Integer>> result = new ArrayList<List<Integer>>();
if (nums == null || nums.length < 4) return result;
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++) {
if (j > i + 1 && nums[j] == num[j - 1]) {
continue;
}

int left = j + 1;
int right = nums.length - 1;
while (left < right) {
int sum = nums[i] + nums[j] + nums[left] + nums[right];
if (sum < target) {
left++;
} else if (sum > target) {
right--;
} else {
ArrayList<Integer> tmp = new ArrayList<Integer>();
left++;
right--;
while (left < right && nums[left] == nums[left - 1]) {
left++;
}
while (left < right && nums[right] == nums[right + 1]) {
right--;
}
}
}
}
}

return result;
}
}``````

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