LeetCode 18. 4Sum

描述
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]]

分析
先排序，然后左右夹逼，复杂度 O(n3)，会超时。
可以用一个 hashmap 先缓存两个数的和，最终复杂度 O(n3)。这个策略也适用于 3Sum 。

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 a = nums.begin(); a < prev(last, 3); ++a) {            for (auto b = next(a); b < prev(last, 2); ++b) {                auto c = next(b);                auto d = prev(last);                while (c < d) {                    if (*a + *b + *c + *d < target) ++c;                    else if (*a + *b + *c + *d > target) --d;                    else {                        result.push_back({*a, *b, *c, *d});                        ++c;                        --d;                    }                }            }        }        sort(result.begin(), result.end());        result.erase(unique(result.begin(), result.end()), result.end());   // 删除重复项        return result;    }};