leetcode.18--------------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.

    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)

1.

class Solution {public:vector<vector<int> > fourSum(vector<int> &num, int target){vector<vector<int> > ret;if (num.size() == 0) return ret;sort(num.begin(), num.end());for (size_t a = 0; a < num.size(); ++a){if (a != 0 && num[a] == num[a - 1]){continue;}for (size_t b = a + 1; b < num.size(); ++b){if (b != a + 1 && num[b] == num[b - 1]){continue;}size_t c = b + 1;size_t d = num.size() - 1;while (c < d){const int sum = num[a] + num[b] + num[c] + num[d];if (sum > target){--d;}else if (sum < target){++c;}else if (c != b + 1 && num[c] == num[c - 1]){++c;}else if (d != num.size() - 1 && num[d] == num[d + 1]){--d;}else{vector<int> result;result.push_back(num[a]);result.push_back(num[b]);result.push_back(num[c]);result.push_back(num[d]);ret.push_back(result);++c;--d;}}}}return ret;}};

2.

class Solution {public:vector<vector<int> > fourSum(vector<int> &num, int target) {// Note: The Solution object is instantiated only once.vector<vector<int>> res;int numlen = num.size();if (num.size()<4)return res;sort(num.begin(), num.end());set<vector<int>> tmpres;for (int i = 0; i < numlen; i++){for (int j = i + 1; j < numlen; j++){int begin = j + 1;int end = numlen - 1;while (begin < end){int sum = num[i] + num[j] + num[begin] + num[end];if (sum == target){vector<int> tmp;tmp.push_back(num[i]);tmp.push_back(num[j]);tmp.push_back(num[begin]);tmp.push_back(num[end]);tmpres.insert(tmp);begin++;end--;}else if (sum<target)begin++;elseend--;}}}set<vector<int>>::iterator it = tmpres.begin();for (; it != tmpres.end(); it++)res.push_back(*it);return res;}};