LeetCode18. 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)

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实现：

class Solution {
public:
void twoSum(vector<int>& nums, int target, int start, int end, vector<vector<int>>& vvi, int z1, int z2)
{
if (start >= end) {
return;
}
if (2 * nums[start] > target || 2 * nums[end] < target) {
return;
}
int i = start;
int j = end;
int sum, x;
while (i < j) {
sum = nums[j] + nums[i];
if (sum == target) {
vvi.push_back(vector<int> {z1, z2, nums[i], nums[j]});
x = nums[i];
while (++i < j && nums[i] == x) {
;
}
x = nums[j];
while (--j > i && nums[j] == x) {
;
}
}
else if (sum < target) {
i++;
}
else {
j--;
}
}
}

void threeSum(vector<int>& nums, int target, int start, int end, vector<vector<int>>& vvi, int z1)
{
if (start + 1 >= end) return;
int max = nums[end];
if (3 * nums[start] > target || 3 * max < target) {
return;
}
int i, z;
for (i = start; i < end - 1; i++) {
z = nums[i];
if (i > start && nums[i - 1] == z) {
continue;
}
if (z + 2 * max < target) {
continue;
}
if (3 * z > target) {
break;
}
if (3 * z == target) {
if (i + 1 < end && nums[i + 2] == z) {
vvi.push_back(vector<int>{z1, z, z, z});
}
break;
}
twoSum(nums, target - z, i + 1, nums.size() - 1, vvi, z1, z);
}
}

vector<vector<int>> fourSum(vector<int>& nums, int target) {
vector<vector<int>> vvi;

if (nums.size() < 4) {
return vvi;
}
std::sort(nums.begin(), nums.end());
int max = nums[nums.size() - 1];
if (4 * nums[0] > target || 4 * max < target)
return vvi;
int i, z;
for (i = 0; i < nums.size(); i++) {
z = nums[i];
if (i > 0 && z == nums[i - 1]) {
continue;
}
if (z + 3 * max < target) {
continue;
}
if (z * 4 > target) {
break;
}
if (4 * z == target) {
if (i + 3 < nums.size() && nums[i + 3] == z) {
vvi.push_back(vector<int>{
z, z, z, z
});
}
break;
}
threeSum(nums, target - z, i + 1, nums.size() - 1, vvi, z);
}
return vvi;
}

};