Leetcode: 4Sum

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)

3Sum的加强版，多一个循环，O(n^3)时间复杂度。注意去重就行了。

class Solution {public:    vector<vector<int> > fourSum(vector<int> &num, int target) {        vector<vector<int> > result;        int size = num.size();        if (size < 4) {            return result;        }                sort(num.begin(), num.end());        int psum = 0, sum = 0;        for (int i = 0; i < size - 3; ++i) {            for (int j = i + 1; j < size - 2; ++j) {                psum = num[i] + num[j];                for (int k = j + 1, h = size - 1; k < h;) {                    sum = psum + num[k] + num[h];                    if (sum < target) {                        ++k;                    }                    else if (sum > target) {                        --h;                    }                    else {                        vector<int> v;                        v.push_back(num[i]);                        v.push_back(num[j]);                        v.push_back(num[k]);                        v.push_back(num[h]);                        result.push_back(v);                                                while (k < h && num[k] == num[++k]);                        while (k < h && num[h] == num[--h]);                    }                }                while (j < size - 2 && num[j] == num[++j]);                --j;            }            while (i < size - 3 && num[i] == num[++i]);            --i;        }                return result;    }};