LeetCode Solutions Blog

Largest Positive Integer That Exists With Its Negative

Number: 2524

Difficulty: Easy

Paid? No

Problem Description

Given an integer array nums that does not contain any zeros, find the largest positive integer k such that -k also exists in the array. Return k if found; otherwise, return -1.

Key Insights

Use a hash set for quick look-up of elements.
Iterate through each number in the array; if the number is positive and its negative is also present, it is a candidate.
Keep track of the maximum valid candidate found.
The array does not contain zero, which simplifies the checks.

Space and Time Complexity

Time Complexity: O(n), where n is the number of elements in the array (using a set for O(1) look-ups). Space Complexity: O(n), for storing elements in the hash set.

Solution

We start by putting all the numbers in a hash set for O(1) look-up time. Then, we iterate through the array and for each positive number k, we check if -k exists in the set. If it does, we update our answer with the maximum value between the current answer and k. Finally, if no valid k is found, we return -1.

Code Solutions

# Define the function to find the largest positive integer that exists with its negative.
def find_max_k(nums):
    # Convert the list to a set for fast lookup.
    num_set = set(nums)
    
    # Initialize the result with -1 (default when no valid k is found).
    max_k = -1
    
    # Iterate through each number in the list.
    for num in nums:
        # Consider only positive numbers.
        if num > 0:
            # Check if the corresponding negative exists in the set.
            if -num in num_set:
                # Update max_k if this number is larger than the current max_k.
                max_k = max(max_k, num)
    return max_k

# Example usage:
print(find_max_k([-1,2,-3,3]))  # Output: 3
print(find_max_k([-1,10,6,7,-7,1]))  # Output: 7
print(find_max_k([-10,8,6,7,-2,-3]))  # Output: -1