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: 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] ]掌握了套路就好了，嗯，都是套路，O(n^3)
class Solution { public: vector<vector<int>> fourSum(vector<int>& nums, int target) { sort(nums.begin(), nums.end()); vector<vector<int> > ans; for (int i = 0; i < nums.size(); ++i) { if (i > 0 && nums[i] == nums[i-1]) continue; for (int j = i + 1; j < nums.size(); ++j) { if (j > i+1 && nums[j] == nums[j-1]) continue; int sum = target - nums[i] - nums[j]; int le = j + 1, ri = nums.size() - 1; while (le < ri) { if (nums[le] + nums[ri] == sum) { vector<int> cur; cur.push_back(nums[i]); cur.push_back(nums[j]); cur.push_back(nums[le]); cur.push_back(nums[ri]); ans.push_back(cur); le++, ri--; while (le < ri && nums[le] == nums[le-1]) le++; while (le < ri && nums[ri] == nums[ri+1]) ri--; } else if (nums[le] + nums[ri] < sum) le++; else ri--; } } } return ans; } };