18. 4Sum

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题目：4Sum

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]]

同样的思路，用C语言怎么都是Runtime error,而JAVA的话就可以通过。

代码：

public class Solution {    public List<List<Integer>> fourSum(int[] nums, int target) {        int i=0,j=0;            List<List<Integer>> ret = new ArrayList<List<Integer>>();            Arrays.sort(nums);            int length=nums.length;                        for(i=0;i<length-3;i++)            {                if(i>0 && nums[i]==nums[i-1]) continue;                        for(j=i+1;j<length-2;j++)            {                if(j>(i+1) && nums[j]==nums[j-1]) continue;                            int s=j+1;            int e=length-1;                        while(s<e)            {                        int sum=nums[i]+nums[j]+nums[s]+nums[e];                        if(sum == target)                        {                                            List<Integer> t=new ArrayList<Integer>();            t.add(nums[i]);            t.add(nums[j]);            t.add(nums[s]);            t.add(nums[e]);            ret.add(t);                        while(s<e && nums[s]==nums[s+1])s++;            while(s<e && nums[e]==nums[e-1])e--;            s++;            e--;                                    }            else if(sum < target)            {            s++;            }            else             {            e--;            }                        }                        }            }                                        return ret;                  }}

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