[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)

跟3Sum一样，先用两层循环，注意去重。

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