Largest Unique Number in Python

In this problem, we need to find the largest number that appears exactly once in a list. If no such number exists, we return -1.

Problem Understanding

Given a list like [5,2,3,6,5,2,9,6,3], we need to ?

• Count the frequency of each number
• Find numbers that appear exactly once
• Return the largest among those unique numbers

Method 1: Using Dictionary for Counting

We can use a dictionary to count occurrences and then find the largest unique number ?

def largest_unique_number(numbers):
    count = {}
    
    # Count frequency of each number
    for num in numbers:
        if num not in count:
            count[num] = 1
        else:
            count[num] += 1
    
    # Find largest number with frequency 1
    largest = -1
    for num, freq in count.items():
        if freq == 1:
            largest = max(num, largest)
    
    return largest

# Test the function
numbers = [5, 2, 3, 6, 5, 2, 9, 6, 3]
result = largest_unique_number(numbers)
print(f"Largest unique number: {result}")

Largest unique number: 9

Method 2: Using Counter from Collections

Python's Counter provides a more concise way to count frequencies ?

from collections import Counter

def largest_unique_counter(numbers):
    count = Counter(numbers)
    unique_numbers = [num for num, freq in count.items() if freq == 1]
    
    return max(unique_numbers) if unique_numbers else -1

# Test with different examples
test_cases = [
    [5, 2, 3, 6, 5, 2, 9, 6, 3],
    [1, 1, 2, 2, 3, 3],
    [7, 4, 9, 4, 7]
]

for numbers in test_cases:
    result = largest_unique_counter(numbers)
    print(f"Input: {numbers}")
    print(f"Largest unique: {result}")
    print()

Input: [5, 2, 3, 6, 5, 2, 9, 6, 3]
Largest unique: 9

Input: [1, 1, 2, 2, 3, 3]
Largest unique: -1

Input: [7, 4, 9, 4, 7]
Largest unique: 9

Method 3: One-Pass Solution

For better efficiency, we can track the largest unique number during counting ?

def largest_unique_optimized(numbers):
    count = {}
    
    # First pass: count frequencies
    for num in numbers:
        count[num] = count.get(num, 0) + 1
    
    # Second pass: find largest unique
    largest = -1
    for num in numbers:
        if count[num] == 1:
            largest = max(largest, num)
    
    return largest

# Test the optimized solution
numbers = [10, 5, 8, 5, 10, 15, 8, 20]
result = largest_unique_optimized(numbers)
print(f"Numbers: {numbers}")
print(f"Largest unique: {result}")

Numbers: [10, 5, 8, 5, 10, 15, 8, 20]
Largest unique: 20

Comparison of Methods

Conclusion

Use Counter for the most readable solution. All methods have O(n) time complexity, making them efficient for finding the largest unique number in any list.