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Technical Interview Problems in Python: Lists

Pair Sum: Naive

We’ll end with a classic interview question: given a list of integers and a “target” integer, **return a pair of indices whose values sum to the target**.

nums = [4, 2, 8, 9, 6] target = 8 pair_sum(nums, target) # nums[1] + nums[4] == 8 # [1, 4] pair_sum(nums, 17) # nums[2] + nums[3] == 17 # [2, 3] pair_sum(nums, 99) # no pair sum exists... # None

**Clarifying Questions:**

- Are there constraints on time or space efficiency?
- No! Any solution will do.

- Can the numbers be negative or 0?
- Yes! Your solution should handle those inputs.

- Does the order of indices matter?
- The earlier index comes first in the return list.

- Do we return all pairs that match a solution?
- No! The first one that your solution finds will do!

### Instructions

**1.**

Write a function `pair_sum()`

, which has two parameters: `nums`

and `target`

.

`pair_sum()`

should return a list containing a pair of indices whose values sum to `target`

.

If no such pair of indices exists, return `None`

.