【LeetCode算法練習（C++）】4Sum

題目：
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: 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]
]

鏈接：4Sum

解法：同3Sum，三層循環嵌套一層指針移動，注意去除相同的集合。時間O(n^3)

class Solution {
public:
    vector<vector<int> > fourSum(vector<int>& nums, int target) {
        set<vector<int> > ans;
        if (nums.size() < 4) return vector<vector<int> > (ans.begin(), ans.end());
        sort(nums.begin(), nums.end());
        for (int i = 0; i < nums.size() - 3; i++) {
            for (int j = i + 1; j < nums.size() - 2; j++) {
                int left = j + 1, right = nums.size() - 1;
                while (left < right) {
                    if (nums[i] + nums[j] + nums[left] + nums[right] == target) {
                        vector<int> v;
                        v.push_back(nums[i]);
                        v.push_back(nums[j]);
                        v.push_back(nums[left]);
                        v.push_back(nums[right]);
                        ans.insert(v);
                        left++;
                        right--;
                    } else if (nums[i] + nums[j] + nums[left] + nums[right] < target) {
                        left++;
                    } else {
                        right--;
                    }
                }
            }
        }
        return vector<vector<int> > (ans.begin(), ans.end());
    }
};

Runtime: 36 ms