【算法】4 Sum 问题

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)

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class Solution {
public:
vector<vector<int>> fourSum(vector<int>& nums, int target) {
vector<vector<int>> results;
sort(nums.begin(), nums.end());
int size = nums.size();
for(int i = 0; i < size - 3; ++i) {
if(i > 0 && nums[i] == nums[i-1]) continue;
if(nums[i]+nums[i+1]+nums[i+2]+nums[i+3]>target) break;
if(nums[i]+nums[size-3]+nums[size-2]+nums[size-1]<target) continue;
vector<int> tmpVec(nums.begin() + i + 1, nums.end());
threeSum(tmpVec, target - nums[i], results, nums[i]);
}
return results;
}
void threeSum(vector<int>& nums, int target, vector<vector<int>> &results, int d) {
int size = nums.size();
for(int i = 0; i < size - 2; ++i) {
if ( i > 0 && (nums[i] == nums[i-1]) ) continue;
int j = i + 1, k = size - 1;
while( j < k) {
while ( j > i + 1 && nums[j] == nums[j - 1]) {
++j;
}
while ( k < size - 1 && nums[k] == nums[k + 1]) {
--k;
continue;
}
if (j >= k) break;
int a = nums[i], b = nums[j], c = nums[k];
int sum = a + b + c;
if (sum == target) {
vector<int> result {d, a, b, c};
results.push_back(result);
++j;
--k;
}
else if(sum < target){
++j;
}
else {
--k;
}
}
}
}
};
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