【LeetCode算法练习（C++）】4Sum

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: 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]
]

链接：4Sum

解法：同3Sum，三层循环嵌套一层指针移动，注意去除相同的集合。时间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