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  • [LeetCode]4Sum

    题目描述:(链接)

    Given an array S of n integers, are there elements abc, 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;
        }
    };
    

      

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  • 原文地址:https://www.cnblogs.com/skycore/p/4853308.html
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