leetcode18. 4Sum

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本文提供了一种解决LeetCode 18题四数之和的方法，通过双指针技巧和去重策略实现了高效求解。代码采用C++编写，详细展示了如何在已排序数组中寻找四个数使它们的和等于目标值。

leetcode18. 4Sum
思路：
关键在于去重，再3sum和2sum中都应该主意去重

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> result = {};
        vector<int> temp = {};
        vector<int> temp1 = {};
        if(nums.size()<4){
            return result; 
        }
        sort(nums.begin(), nums.end());
        for(int i=0;i<nums.size();i++){

            for(int j=nums.size() - 1;j>i;j--){
                int k=i,l=j;
                while(l>k){
                    temp = twoSum(nums, target - nums[i] - nums[j],k,l);
                    if(temp.size()==4){
                        temp1 = {};
                        temp1.push_back(nums[i]);//cout<<temp[0]<<temp[1]<<temp[2]<<temp[3]<<endl;
                        temp1.push_back(nums[j]);
                        temp1.push_back(temp[0]);
                        temp1.push_back(temp[1]);
                        result.push_back(temp1);
                        k=temp[2];
                        l=temp[3];
                    }else{
                        break;
                    }
                    while(nums[k] == nums[k+1] && l>k) k++;
                    while(nums[l] == nums[l-1] && l>k) l--;
                }

              while(nums[j-1] == nums[j] && j>i) j--; 
            }
            while(nums[i] == nums[i+1] && i<nums.size()) i++;


        }
        return  result;
    }


    vector<int> twoSum(vector<int>& nums, int target,int i,int j) {
        if(j-i<=2){
            return vector<int>(); 
        }
        vector<int> numsCopy(nums.begin()+i+1,nums.begin()+j);//cout<<numsCopy<<endl;
        //numsCopy.erase(numsCopy.begin() + i);
        //numsCopy.erase(numsCopy.begin() + j-1);

        //copy(numsCopy.begin(), numsCopy.end(), ostream_iterator<int>(cout, ""));cout<<endl;


        vector<int>::iterator iter = numsCopy.begin();
        vector<int>::iterator iter_end = numsCopy.end();
        iter_end--;
        while (iter != iter_end && (*iter + *iter_end) != target) {

            //cout << *iter << endl;
            if (*iter + *iter_end < target) {
                ++iter;
            }
            else if (*iter + *iter_end > target) {
                iter_end--;
            }
            else {
                break;
            }
        }
        vector<int> ret = {};
        if(iter == iter_end){
            return ret;
        }
        ret.push_back(*iter);

        ret.push_back(*iter_end);

        ret.push_back(iter - numsCopy.begin() +i+1);


        ret.push_back(iter_end - numsCopy.begin() +i+1);

        return ret;
    }

};