LeetCode: 4 Sum

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.

    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)

#include<iostream>#include<vector>#include<algorithm>#include<map>using namespace::std;class Solution {public:long RSHash(string str){int a = 876283;int b = 21382;long hash = 0;for(int i = 0; i < str.size(); i++){hash = a * hash + str[i];a = a * b;}return hash;}    vector<vector<int> > fourSum(vector<int> &num, int target) {        // Start typing your C/C++ solution below        // DO NOT write int main() functionint i = 0, j = i + 1, l = num.size() - 1;string str;int key = 0;vector< vector<int> > result;vector<int> set;map<long, int> mymap;if(num.size() < 4){return result;}sort(num.begin(), num.end());for(int x = 0; x < num.size() - 3; x++){int a = num[x];for(i = x + 1; i < num.size() - 2; i++){int b = num[i];for(j = i + 1, l = num.size() - 1; j < l; ){int c = num[j];int d = num[l];if(a + b + c + d < target){j++;}else if(a + b + c + d > target){l--;}else{str.clear();str += a;str += b;str += c;str += d;key = RSHash(str);if(mymap.find(key) == mymap.end()){mymap[key] = 1;set.push_back(a);set.push_back(b);set.push_back(c);set.push_back(d);result.push_back(set);set.clear();}j++;}}}}return result;            }};int main(){vector<int> aa;aa.push_back(-1);aa.push_back(0);aa.push_back(1);aa.push_back(2);aa.push_back(-1);aa.push_back(-4);Solution ss;ss.fourSum(aa,4);//ss.print(ss.threeSum(aa));}

Round 2:

class Solution {public:    vector<vector<int> > fourSum(vector<int> &num, int target) {            vector<vector<int> > result;        if(num.size() < 4)            return result;        std::sort(num.begin(), num.end());        for(int i = 0; i < num.size()-3; i++)        {            for(int j = i+1; j < num.size()-2; j++)            {                int l = j+1, r = num.size()-1;                while(l < r)                {                    if(num[i] + num[j] + num[l] + num[r] == target)                    {                        vector<int> cur;                        cur.push_back(num[i]);                        cur.push_back(num[j]);                        cur.push_back(num[l]);                        cur.push_back(num[r]);                        result.push_back(cur);                        while(l+1 < r && num[l] == num[l+1])                            l++;                        while(r-1 > l && num[r] == num[r-1])                            r--;                        l++;                        r--;                    }                    else if(num[i] + num[j] + num[l] + num[r] < target)                    {                        l++;                    }                    else                        r--;                }                while(j+1 < num.size()-2 && num[j] == num[j+1])                    j++;            }            while(i+1 < num.size()-3 && num[i] == num[i+1])                i++;        }        return result;    }};