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)

Solution:

Code:

<span style="font-size:14px;">class Solution {public:    vector<vector<int> > fourSum(vector<int> &num, int target) {        const int length = num.size();        vector<vector<int> > results;        sort(num.begin(), num.end());        for (int i = 0; i < length-3; ++i) {            for (int j = i+1; j < length-2; ++j) {                int m = j+1, n = length-1;                while (m < n) {                    if (num[i]+num[j]+num[m]+num[n] == target) {                        vector<int> result;                        result.push_back(num[i]);                        result.push_back(num[j]);                        result.push_back(num[m]);                        result.push_back(num[n]);                        results.push_back(result);                        while (m+1 < n && num[m+1] == num[m]) ++m;                        ++m;                        while (n-1 > m && num[n-1] == num[n]) --n;                        --n;                    } else if (num[i]+num[j]+num[m]+num[n] < target) {                        while (m+1 < n && num[m+1] == num[m]) ++m;                        ++m;                    } else {                        while (n-1 > m && num[n-1] == num[n]) --n;                        --n;                    }                }                while (j+1 < length-2 && num[j+1] == num[j]) ++j;            }            while (i+1 < length-3 && num[i+1] == num[i]) ++i;        }        return results;    }};</span>