Lists

Lists

A Python list is an ordered, mutable sequence of any type. Lists are created with square brackets.

Creating and Accessing

nums = [3, 1, 4, 1, 5]
nums[0]   # 3
nums[-1]  # 5
nums[1:3] # [1, 4]

Mutating Lists

nums.append(9)       # add to end
nums.insert(0, 0)    # insert at index
nums.pop()           # remove last
nums.pop(0)          # remove at index
nums.remove(1)       # remove first occurrence of value
nums.sort()          # sort in-place
nums.reverse()       # reverse in-place

Useful Built-ins

len([1, 2, 3])       # 3
sum([1, 2, 3])       # 6
min([3, 1, 2])       # 1
max([3, 1, 2])       # 3
sorted([3, 1, 2])    # [1, 2, 3]  (returns new list)
list(range(5))       # [0, 1, 2, 3, 4]

Checking Membership

3 in [1, 2, 3]   # True
4 in [1, 2, 3]   # False

Your Task

Implement two_sum(nums, target) that:

• Returns True if any two distinct elements in nums sum to target
• Returns False otherwise

Hint: use a set to track which numbers you've seen. A set is an unordered collection of unique elements — created with set() — that supports O(1) lookups via in, and .add() to insert elements. (Sets also support operations like | for union and & for intersection.)