[LeetCode]4Sum

標籤：

題目描述:(連結)

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)

解題思路:

排序，左右夾逼！但是時間複雜度O(n^3)，哎，待續！

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;    }};

　　

[LeetCode]4Sum