lintcode/4-sum_2(AC).cpp at master · siXence/lintcode · GitHub

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// Without hashing
#include <algorithm>
using namespace std;

class Solution {
public:
    /**
     * @param numbers: Give an array numbersbers of n integer
     * @param target: you need to find four elements that's sum of target
     * @return: Find all unique quadruplets in the array which gives the sum of
     *          zero.
     */
    vector<vector<int> > fourSum(vector<int> nums, int target) {
        ans.clear();
        vector<int> &a = nums;
        vector<int> v(4);
        int n = nums.size();
        int i1, i2, i3, i4;

        sort(a.begin(), a.end());
        i1 = 0;
        while(i1 < n) {
            i4 = n - 1;
            while (i4 > i1) {
                i2 = i1 + 1;
                i3 = i4 - 1;
                while (i2 < i3) {
                    if (a[i1] + a[i2] + a[i3] + a[i4] < target) {
                        ++i2;
                    } else if (a[i1] + a[i2] + a[i3] + a[i4] > target) {
                        --i3;
                    } else {
                        v[0] = a[i1];
                        v[1] = a[i2];
                        v[2] = a[i3];
                        v[3] = a[i4];
                        ans.push_back(v);
                        i2 = nextPos(a, n, i2);
                    }
                }
                i4 = lastPos(a, i1, i4);
            }
            i1 = nextPos(a, n, i1);
        }
        return ans;
    }
private:
    vector<vector<int> > ans;

    int lastPos(vector<int> &a, int n, int i) {
        int val = a[i];
        while (i > n && a[i] == val) {
            --i;
        }
        return i;
    }

    int nextPos(vector<int> &a, int n, int i) {
        int val = a[i];
        while (i < n && a[i] == val) {
            ++i;
        }
        return i;
    }
};