FAANG · LeetCode #1

Two Sum — three-stage drill

1The ProblemEasy

Two Sum

Return indices of the two numbers that add up to a target.

Examples

[3,2,4], target=6→ [1,2][3,2,4], target=10→ none

constraintExactly one solution; can't use the same element twice.

2The PatternTIME O(n) · SPACE O(n)

Arrays & Hashing

Trade space for time: a hash map turns 'have I seen this?' into an O(1) lookup.

Problems using this pattern

3The SolutionRESULT → [1, 2]

Step through it

seen = {} (value → index). target = 6

Step 1 / 4

Python▎look up the complement

1def twoSum(nums, target):
2    seen = {}                 # value -> index
3    for i, x in enumerate(nums):
4        need = target - x
5        if need in seen:
6            return [seen[need], i]
7        seen[x] = i

why it worksThe brute-force pair check is O(n²). Instead, store each value's index as you go; for the current x, its partner must equal target − x — and a hash map answers 'have I seen target − x?' in O(1). One pass, O(n).

Arrow keys to navigate · escape to reset

All problems by pattern

Two Pointers

Sliding Window

Prefix Sum

Monotonic Deque

Shortest Subarray with Sum at Least K

Binary Search

Arrays & Hashing

Monotonic Stack

Intervals

Two Sum — three-stage drill

Two Sum

Arrays & Hashing

Step through it

All problems by pattern

Two Pointers

Sliding Window

Prefix Sum

Monotonic Deque

Binary Search

Arrays & Hashing

Monotonic Stack

Intervals

Heap / Top-K

Backtracking

Recursive Descent