你的for循环里写的好像全是i++。。。

#include<iostream>
#include<vector>
#include<unordered_map>
#include<algorithm>
using namespace std;


class Solution {
public:
    vector<vector<int>> For4Sum(vector<int> &nums,int target) {
        vector<vector<int>> result;
        unordered_map<int,vector<pair<int,int>>> map;

        sort(nums.begin(), nums.end());

        if (nums.size() < 4) return result;

        for (int i = 0; i < nums.size()-1; i++) {
            for (int j = i + 1; j < nums.size(); i++) {
                map[nums[i]+nums[j]].push_back(pair<int,int>(i,j));
            }
        }

        for (int i = 0; i < nums.size() - 1; i++) {
            for (int j = 0; j < nums.size(); i++) {

                int key = target - nums[i] - nums[j];
                if (map.find(key) != map.end()) {
                    for (int k = 0; k < map[key].size(); k++) {
                        if (i < map[key][k].first) {
                            result.push_back({ nums[i], nums[j], nums[map[key][k].first],nums[map[key][k].second] });
                        }
                    }
                }

            }
        }

        sort(result.begin(), result.end());
        result.erase(unique(result.begin(),result.end()),result.end());

        return result;
    }


};


int main() {

    Solution a;
    vector<int> test;
    int target;
    int temp = 0;
    cout << "input target" << endl;
    cin >> target;

    cout << "input nums" << endl;
    while (cin >> temp) {//ctrl+z  
        test.push_back(temp);
    }


    vector<vector<int>> result; 
    result=a.For4Sum(test, target);

    for (int i = 0; i < result.size(); i++) {
        for (int j = 0; j < result[i].size(); j++)
            cout << result[i][j] << " " << endl;
        cout << endl;
    }


    system("pause");
    return 0;
} 

自己出来了，是i j问题，有2个循环那里，j写成了i。改好后就可以跑了，哈哈哈哈。这是我写的4个数之和。leetcode的题目，欢迎参考讨论！

 #include<iostream>
#include<vector>
#include<unordered_map>
#include<algorithm>
using namespace std;


class Solution {
public:
    vector<vector<int>> For4Sum(vector<int> &nums,int target) {
        vector<vector<int>> result;
        unordered_map<int,vector<pair<int,int>>> map;

        sort(nums.begin(), nums.end());

        if (nums.size() < 4) return result;

        for (int i = 0; i < nums.size()-1; i++) {
            for (int j = i + 1; j < nums.size(); j++) {
                map[nums[i]+nums[j]].push_back(pair<int,int>(i,j));
            }
        }

        for (int i = 0; i < nums.size() - 1; i++) {
            for (int j = 0; j < nums.size(); j++) {

                int key = target - nums[i] - nums[j];
                if (map.find(key) != map.end()) {
                    for (int k = 0; k < map[key].size(); k++) {
                        if (i < map[key][k].first) {
                            result.push_back({ nums[i], nums[j], nums[map[key][k].first],nums[map[key][k].second] });
                        }
                    }
                }

            }
        }

        sort(result.begin(), result.end());
        result.erase(unique(result.begin(),result.end()),result.end());

        return result;
    }


};


int main() {

    Solution a;
    vector<int> test;
    int target;
    int temp = 0;
    cout << "input target" << endl;
    cin >> target;

    cout << "input nums" << endl;
    while (cin >> temp) {//ctrl+z  
        test.push_back(temp);
    }


    vector<vector<int>> result; 
    result=a.For4Sum(test, target);

    for (int i = 0; i < result.size(); i++) {
        for (int j = 0; j < result[i].size(); j++)
            cout << result[i][j] << " " ;
        cout << endl;
    }


    system("pause");
    return 0;
}

第二个循环j忘记i+1，然后要从小到大排序的话，经过测试，一下是最终版。我觉得我写的比网上的那个好些 哈哈哈。

题目：
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:
1.Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a≤b≤c≤d)
2.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)

这是我写的：

 #include<iostream>
#include<vector>
#include<unordered_map>
#include<algorithm>
using namespace std;


class Solution {
public:
    vector<vector<int>> For4Sum(vector<int> &nums,int target) {
        vector<vector<int>> result;
        unordered_map<int,vector<pair<int,int>>> map;

        sort(nums.begin(), nums.end());

        if (nums.size() < 4) return result;

        for (int i = 0; i < nums.size()-1; i++) {
            for (int j = i + 1; j < nums.size(); j++) {
                map[nums[i]+nums[j]].push_back(pair<int,int>(i,j));
            }
        }

        for (int i = 0; i < nums.size() - 1; i++) {
            for (int j = i+1; j < nums.size(); j++) {

                int key = target - nums[i] - nums[j];
                if (map.find(key) != map.end()) {
                    for (int k = 0; k < map[key].size(); k++) {
                        if ( j < map[key][k].first) {
                            result.push_back({ nums[i], nums[j], nums[map[key][k].first],nums[map[key][k].second] });
                        }
                    }
                }

            }
        }

        sort(result.begin(), result.end());
        result.erase(unique(result.begin(),result.end()),result.end());

        return result;
    }


};


int main() {

    Solution a;
    vector<int> test;
    int target;
    int temp = 0;
    cout << "input target" << endl;
    cin >> target;

    cout << "input nums" << endl;
    while (cin >> temp) {//ctrl+z  
        test.push_back(temp);
    }


    vector<vector<int>> result; 
    result=a.For4Sum(test, target);

    for (int i = 0; i < result.size(); i++) {
        for (int j = 0; j < result[i].size(); j++)
            cout << result[i][j] << " ";
        cout << endl;
    }


    system("pause");
    return 0;
}

这是网上的版本：

 class Solution {
public:
    vector<vector<int> > fourSum(vector<int> &nums, int target) {
        vector<vector<int>> result;
        if (nums.size() < 4) return result;
        sort(nums.begin(), nums.end());

        unordered_map<int, vector<pair<int, int> > > cache;
        for (size_t a = 0; a < nums.size(); ++a) {
            for (size_t b = a + 1; b < nums.size(); ++b) {
                cache[nums[a] + nums[b]].push_back(pair<int, int>(a, b));
            }
        }

        for (int c = 0; c < nums.size(); ++c) {
            for (size_t d = c + 1; d < nums.size(); ++d) {
                const int key = target - nums[c] - nums[d];
                if (cache.find(key) == cache.end()) continue;

                const auto& vec = cache[key];
                for (size_t k = 0; k < vec.size(); ++k) {
                    if (c <= vec[k].second)
                        continue; // 有重叠

                    result.push_back( { nums[vec[k].first],
                            nums[vec[k].second], nums[c], nums[d] });
                }
            }
        }
        sort(result.begin(), result.end());
        result.erase(unique(result.begin(), result.end()), result.end());
        return result;
    }
};