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4Sum.cpp
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54 lines (47 loc) · 1.75 KB
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/**
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)
*/
class Solution {
public:
vector<vector<int> > fourSum(vector<int> &num, int target) {
vector<vector<int> > res;
if (num.empty() || num.size() < 4) return res;
std::sort(num.begin(), num.end());
vector<int> quadruplet(4);
int n = num.size();
for (int i = 0; i < n; ++i) {
if (i >0 && num[i] == num[i-1]) continue;
for (int j = i +1; j < n; ++j) {
if (j >i+1 && num[j] == num[j-1]) continue;
int l = j+1, r = n-1;
while (l <r) {
int sum = num[i] + num[j] + num[l] + num[r];
if (sum == target) {
quadruplet[0] = num[i];
quadruplet[1] = num[j];
quadruplet[2] = num[l];
quadruplet[3] = num[r];
res.push_back(quadruplet);
l ++;
r --;
while (l <r && num[l] == num[l-1]) l++;
while (l <r && num[r] == num[r+1]) r--;
}
else if (sum < target) l ++;
else r--;
}
}
}
return res;
}
};