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LeetCodeRelated practiceStack / Hash Map LookupSolved Nov 7, 2025
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Go step by step: try it yourself before the video and the visual. Progress saves on this device.
Community-reported · source · updated 2026-10-08. ● asked in the last 6 months.
Step 1
Know these before you start
Tick what you already know
Concept videoStep 2
Try it on paper
5–10 minutes, no code yet
Given a string S containing only the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. A valid string requires opening brackets to be closed by the same type of bracket and in the correct order.
Step 6
Get it accepted
All test cases pass
Step 7
Review it so it sticks
Re-solve from memory after
Reading the solution first feels like progress, but it makes the next similar problem — and the interview version — much harder, because you skipped the part where you figure it out. Give it an honest 20–30 minutes. Stuck? Re-read the pattern, watch the concept video, or try the brute force first.
Hidden: approach · solution code · optimal solution