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Top Algorithm Interview Questions Explained - Valid Parentheses - Stacks, Easy In these series I'm going to solve many Programming Interview questions related to algorithms and explain my thought process. I will try to be detailed and show you how a programmer thinks when confronted with such questions. This one is about Stacks and is Easy: Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input string is valid if: - Open brackets must be closed by the same type of brackets. - Open brackets must be closed in the correct order. - Every close bracket has a corresponding open bracket of the same type.