Leetcode||18. 4Sum

18. 4Sum

Total Accepted: 94146
Total Submissions: 371949
Difficulty: Medium
Contributors: Admin

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]
]

这题和15题，16题类似，排序，前后夹逼可以获取全部情况，就是一个动态规划
代码如下：

public class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
List<List<Integer>> res = new ArrayList<List<Integer>>();
int length = nums.length;
if (length<4) {
return res;
}
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, end = length-1;
while (begin < end) {
int sum = nums[i]+nums[j]+nums[begin]+nums[end];
if (sum == target) {
List<Integer> temp = new ArrayList<Integer>();
temp.add(nums[i]);
temp.add(nums[j]);
temp.add(nums[begin]);
temp.add(nums[end]);
res.add(temp);
begin++;
end--;
while (begin<end && nums[begin]==nums[begin-1]) begin++;
while (begin<end && nums[end]==nums[end+1]) end--;
} else if (sum > target) {
end--;
while (begin<end && nums[end]==nums[end+1]) end--;
} else {
begin++;
while (begin<end && nums[begin]==nums[begin-1]) begin++;
}
}
}
}
return res;
}
}