#3 Easy Time O(n) Space O(n) Animated

Two Sum

Given an array of integers and a target, return indices of two numbers that add up to target.

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1 Predict
2 Trace
3 Explain
4 Implement

Pattern Hashing for constant-time memory

Trade one linear scan for a small lookup structure so each value is handled once.

Approach: Hash Map — One Pass

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Core Insight

Trade one linear scan for a small lookup structure so each value is handled once.

Common Pitfall

Do not overwrite information you still need before checking the current value.

Description

Given an array of integers and a target, return indices of two numbers that add up to target.

Constraints

Time Complexity: O(n)
Space Complexity: O(n)

Solution — Hash Map — One Pass

TimeO(n) SpaceO(n)

Implementation Plan

Translate the invariant into these small, testable moves.

Create the lookup structure.
Scan each value once.
Check the needed relationship before updating state.
Return as soon as the answer is determined.

def twoSum(nums, target):
    seen = {}                    # line 1
    for i, num in enumerate(nums):  # line 2
        complement = target - num   # line 3
        if complement in seen:      # line 4
            return [seen[complement], i]  # line 5
        seen[num] = i              # line 6
    return []                      # line 7

Valid Anagram

Group Anagrams

Two Sum

Steps

Description

Constraints

Tags

Solution — Hash Map — One Pass