4sum java_Java [leetcode 18]4Sum

问题描述：

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)

解题思路：

与之前的3sum相类似，只不过在外边多了一套循环，时间复杂度为O(n^3)

代码如下：

public class Solution {

List> ans = new ArrayList>();

public List> fourSum(int[] nums, int target) {

int length = nums.length;

int tmp;

if (nums == null || length < 4)

return ans;

Arrays.sort(nums);

for (int i = 0; i < length - 3; i++) {

if (i > 0 && nums[i] == nums[i - 1])

continue;

for (int j = i + 1; j < length - 2; j++) {

if (j > i + 1 && nums[j] == nums[j - 1])

continue;

int begin = j + 1;

int end = length - 1;

while (begin < end) {

tmp = nums[begin] + nums[end] + nums[i] + nums[j];

if (tmp == target) {

List list = new ArrayList();

list.add(nums[i]);

list.add(nums[j]);

list.add(nums[begin]);

list.add(nums[end]);

ans.add(list);

while (begin < end && nums[begin + 1] == nums[begin])

begin++;

begin++;

while (begin < end && nums[end - 1] == nums[end])

end--;

end--;

} else if (tmp > target)

end--;

else

begin++;

}

}

}

return ans;

}

}