# int32bit / leetcode

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solve.cpp

## 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扩展

## Code

```class Solution {
public:
vector<vector<int>> fourSum(vector<int> &num, int target)
{
vector<vector<int>> result;
int n = num.size();
if (n < 4)
return result;
sort(num.begin(), num.end());
for (int i = 0; i < n - 3; ++i) {
if (i > 0 && num[i] == num[i - 1])
continue;
vector<int> n(num.begin() + i + 1, num.end());
vector<vector<int>> ret = threeSum(n, target - num[i]);
for (auto v : ret)
{
v.insert(v.begin(), num[i]);
result.push_back(v);
}
}
return result;
}
vector<vector<int>> threeSum(vector<int> &num, int target) {
//	sort(num.begin(), num.end());
vector<vector<int>> result;
auto n = num.size();
size_t i;
for (i = 0; i < n; ++i) {
if (i > 0 && num[i] == num[i - 1])
continue;
int a = num[i];
int low = i + 1, high = n - 1;
while (low < high) {
int b = num[low];
int c = num[high];
if (a + b + c == target) {
vector<int> v = {a, b, c};
result.push_back(v);
while (low + 1 < n && num[low] == num[low + 1]) low++;
while (high - 1 >= 0 && num[high] == num[high - 1]) high--;
low++;
high--;
} else if (a + b + c > target) {
while (high - 1 >= 0 && num[high] == num[high - 1]) high--;
high--;
} else {
while (low + 1 < n && num[low] == num[low + 1]) low++;
low++;
}
}
}
return result;
}
};```
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