Two Sum / Hashmap

Problem

Return indices of two numbers summing to a target.

Input / Output

• Input: an integer array nums and an integer target.
• Output: the indices [i, j] of the two numbers that add up to target.

Constraints

• 2 ≤ n ≤ 10^4
• Exactly one valid pair is guaranteed.

Example

[2,7,11,15], 9 -> [0,1]