Meta Data#
Difficulty: easy First Attempt: 2025-08-06
- Submissions: Wrong Answer ×4 → Accepted
Approach#
Find the largest positive k such that -k is also in the array.
Put positives and absolute values of negatives into two separate sets; values in both are valid k, and the answer is the largest.
My Attempts#
Attempts 1 and 2: absolute values only#
for num in nums:
if num < 0:
num *= -1
if num not in exist:
exist.add(num)
else:
max_num = max(max_num, num)
Every number became its absolute value, and a second sighting counted as a pair. The problem: same-sign duplicates also count, so [3, 3] returns 3 even though -3 is not in the array. The second attempt only added “return -1 when nothing is found” and did not fix this.
Attempts 3 and 4: two sets, signs mixed up#
Next I used one set for positives and one for negatives, but the signs were never handled right: the third attempt put raw negatives into exist_pos and positives into exist_neg, so one set held only negatives and the other only positives and the intersection was always empty; the fourth negated the positives, so both sets held negatives and the intersection was found, but it returned -k. The final version stores “negatives as absolute values in exist_neg, positives as-is in exist_pos” and passed.
Solution#
class Solution:
def findMaxK(self, nums: List[int]) -> int:
pos = {x for x in nums if x > 0}
neg = {-x for x in nums if x < 0}
both = pos & neg
return max(both) if both else -1
Alternative: sort and use two pointers#
After sorting, start the left pointer at the smallest negative and the right pointer at the largest positive: equal absolute values give the answer; if the left side’s absolute value is larger, move left rightwards, otherwise move right leftwards. Time O(n log n), space O(1).
Complexity#
Time O(n), space O(n).
Mistakes to Avoid#
- Looking only at absolute values: same-sign duplicates are mistaken for pairs.
- Inconsistent signs between the two sets: the intersection is empty, or the answer comes back negative.
Interview Explanation#
I put positives in one set and absolute values of negatives in another; their intersection is the numbers present with both signs, and I return the largest, or -1 if it is empty. Absolute values alone do not work, because same-sign duplicates would look like a pair. Time O(n).