Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.
Note:
The solution set must not contain duplicate quadruplets.
Example:
Given array nums = [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] ]
AC code:
class Solution {
public:
vector<vector<int>> fourSum(vector<int>& nums, int target) {
int len = nums.size();
vector<vector<int>> v;
if (len < 4) return v;
sort(nums.begin(), nums.end());
for (int i = 0; i < len-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[len-1] + nums[len-2] + nums[len-3] < target) continue;
for (int j = i+1; j < len -2; ++j) {
if (j > i+1 && nums[j] == nums[j-1]) continue;
if (nums[i] + nums[j] + nums[j+1] + nums[j+2] > target) break;
if (nums[i] + nums[j] + nums[len-2] + nums[len-1] < target) continue;
int left = j + 1;
int right = len - 1;
while (left < right) {
int sum = nums[i] + nums[j] + nums[left] + nums[right];
if (sum > target) {
right--;
} else if (sum < target) {
left++;
} else {
v.push_back(vector<int>{nums[i], nums[j], nums[left], nums[right]});
// don‘t use while statement.
do {
left++;
} while (left < right && nums[left] == nums[left-1]);
do {
right--;
} while (left < right && nums[right] == nums[right+1]);
}
}
}
}
return v;
}
};
Runtime: 20 ms, faster than 75.88% of C++ online submissions for 4Sum.
原文：https://www.cnblogs.com/ruruozhenhao/p/9738737.html