2441. Largest Positive Integer That Exists With Its Negative

Thu, 02 May 2024 10:28:05 +0800

Given a list containing integers, find the largest positive integer that its negative also exists in the list.

Intuition

There are two ways to solve the problem.

One way is to use two sum, we find the all pairs of integers such that their sum is zero and the one is the negative of the other.
The second way is to use two pointer, we move left and right pointer utils their absolute values are equal.

Approach 1: two sum

Similar to Two Sum, for each num in nums, we store its negative -num in the hash table, however, notice that the added term can be determined by num, we can use a set instead.

Complexity

Time complexity: iterate the list once.
$$O(n)$$
Space complexity: use a set to store numbers.
$$O(n)$$

Code

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class Solution {
public:
 int findMaxK(vector<int>& nums) {
 set<int> s;
 int result = -1;
 for (int num: nums) {
 if (s.find(-num) != s.end()) {
 result = max(result, abs(num));
 } else {
 s.insert(num);
 }
 }
 return result;
 }
};

Approach 2: two pointer

We first sort the lists, then we use left=0 and right=length(nums) pointer to iterate the list, there are three cases:

if nums[left] == -nums[right], then we directly return nums[right] since this is the largest one (note that the list is sorted)
if nums[left] < -nums[right], then we update left to left + 1 since (nums[left], -nums[left]) cannot be found in the list.
if nums[left] > -nums[right], then we update right to right - 1 since (-nums[right], nums[right]) cannot be found in the list.

Complexity

Time complexity: sort the array and iterate the list once.
$$O(n\log n)$$
Space complexity: no extra spaces are needed.
$$O(1)$$

Code

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class Solution {
public:
 int findMaxK(vector<int>& nums) {
 sort(nums.begin(), nums.end());
 int left = 0, right = nums.size() - 1;
 while (left <= right) {
 if (nums[left] == -nums[right]) return nums[right];
 else if (nums[left] < -nums[right]) ++left;
 else --right;
 }
 return -1;
 }
};
``

# References
- [Leetcode](https://leetcode.com/problems/largest-positive-integer-that-exists-with-its-negative/description/)
- [Leetcode Two Sum](https://leetcode.com/problems/two-sum/)

List on Jiang Yi(姜祎)'s Homepage

2441. Largest Positive Integer That Exists With Its Negative

Intuition

Approach 1: two sum

Complexity

Code

Approach 2: two pointer

Complexity

Code