Leetcode[18]-4Sum

Link: https://leetcode.com/problems/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:
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.

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分析：和上道题有点类似，多了一层循环，注意该循环的条件就行.

class Solution {public:    vector<vector<int> > fourSum(vector<int>& nums, int target) {        vector<vector<int> > result;            int n = nums.size();                sort(nums.begin(),nums.end());        for(int i = 0; i < n; i++) {            if(i>0 && nums[i] == nums[i-1]) continue;            for(int j = i+1; j < n; j++){                if(j > i+1 && nums[j]== nums[j-1]) continue;                fourNumber(nums,result,i,j,target);            }        }        return result;    }    void fourNumber(vector<int> & nums,vector<vector<int> > &results, int curIndex1,int curIndex2,int target){        int i = curIndex2 + 1;        int j = nums.size()-1;        while(i<j) {            if(nums[curIndex1] + nums[curIndex2] + nums[i] + nums[j] < target ) i++;            else if(nums[curIndex1] + nums[curIndex2] + nums[i] + nums[j] > target ) j--;            else {                vector<int> vec;                vec.push_back(nums[curIndex1]);                vec.push_back(nums[curIndex2]);                vec.push_back(nums[i]);                vec.push_back(nums[j]);                results.push_back(vec);                i++,j--;                while(i < j && nums[i] == nums[i-1])i++;                while(j > i && nums[j] == nums[j+1])j--;            }        }    }};