【刷题日记】哈希表-四数之和-L18-Medium
给你一个由 n 个整数组成的数组 nums ，和一个目标值 target 。请你找出并返回满足下述全部条件且不重复的四元组 [nums[a], nums[b], nums[c], nums[d]] （若两个四元组元素一一对应，则认为两个四元组重复）：
学习点
代码
双指针法，需要考虑到四数之和可能会超过 int 范围：
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| class Solution {
public:
vector<vector<int>> fourSum(vector<int>& nums, int target) {
vector<vector<int>> res;
sort(nums.begin(), nums.end());
if (nums.size() < 4)
return res;
size_t nums_size = nums.size();
for (size_t i = 0; i < nums_size; i++)
{
if (i >= 1 && nums[i] == nums[i - 1])
continue;
for (size_t j = i + 1; j < nums_size; j++)
{
if (j >= i + 2 && nums[j] == nums[j - 1])
continue;
int left = j + 1;
int right = nums_size - 1;
while (left < right)
{
while (left < right &&
(long)nums[i] + (long)nums[j] + (long)nums[left] + (long)nums[right] > target)
--right;
while (left < right &&
(long)nums[i] + (long)nums[j] + (long)nums[left] + (long)nums[right] < target)
++left;
if (left >= right)
break;
else if ((long)nums[i] + (long)nums[j] + (long)nums[left] + (long)nums[right] == target)
{
res.emplace_back(vector<int>{nums[i], nums[j], nums[left], nums[right]});
++left;
while (left < right && nums[left] == nums[left - 1])
++left;
--right;
while (left < right && nums[right] == nums[right + 1])
--right;
}
}
}
}
return res;
}
};
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优化的解法：
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| class Solution {
public:
vector<vector<int>> fourSum(vector<int>& nums, int target) {
vector<vector<int>> result;
sort(nums.begin(), nums.end());
for (int k = 0; k < nums.size(); k++) {
if (nums[k] > target && nums[k] >= 0) {
break;
}
if (k > 0 && nums[k] == nums[k - 1]) {
continue;
}
for (int i = k + 1; i < nums.size(); i++) {
if (nums[k] + nums[i] > target && nums[k] + nums[i] >= 0) {
break;
}
if (i > k + 1 && nums[i] == nums[i - 1]) {
continue;
}
int left = i + 1;
int right = nums.size() - 1;
while (right > left) {
if ((long) nums[k] + nums[i] + nums[left] + nums[right] > target) {
right--;
} else if ((long) nums[k] + nums[i] + nums[left] + nums[right] < target) {
left++;
} else {
result.push_back(vector<int>{nums[k], nums[i], nums[left], nums[right]});
while (right > left && nums[right] == nums[right - 1]) right--;
while (right > left && nums[left] == nums[left + 1]) left++;
right--;
left++;
}
}
}
}
return result;
}
};
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