[LeetCode]4Sum

题目描述:(链接)

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)

解题思路:

排序，左右夹逼！但是时间复杂度O(n^3)，哎，待续！

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> result;
        if (nums.size() < 4) { return result;}
        sort(nums.begin(), nums.end());
        
        auto last = nums.end();
        
        for (auto i = nums.begin(); i != last - 3; ++i) {
            for (auto j = i + 1; j != last - 2; ++j) {
                auto k = j + 1;
                auto m = last - 1;
                while (k < m) {
                    int sum = *i + *j + *k + *m;
                    if (sum < target) {
                        ++k;
                    } else if(sum > target) {
                        --m;
                    } else {
                        result.push_back({*i, *j, *k, *m});
                        ++k;
                        --m;
                    }
                }

            }
        }
        sort(result.begin(), result.end());
        result.erase(unique(result.begin(), result.end()), result.end());
        
        return result;
    }
};