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Stack

Valid Parentheses

easy

DESCRIPTION (credit Leetcode.com)

Given an input string s consisting solely of the characters '(', ')', '{', '}', '[' and ']', determine whether s is a valid string. A string is considered valid if every opening bracket is closed by a matching type of bracket and in the correct order, and every closing bracket has a corresponding opening bracket of the same type.

Example 1:

Inputs:

s = "(){({})}"

Output:

True

Example 2:

Inputs:

s = "(){({}})"

Output:

False

Explanation

The input string can contain nested brackets which must adhere to a Last-In, First-Out ordering (i.e. if our input string is {[, then the closing ] must come before before the closing }). For this reason, our solution will use a stack.

The solution iterates through the string. Whenever it encounters an opening bracket, the bracket is pushed onto the stack. When it encounters a closing bracket, we check to see if it is the corresponding closing bracket for the opening bracket at the top of the stack. If it is, we pop from the stack (because we have found a matching parantheses) and continue iterating. If it isn't, the string is invalid, and we return False.

The string is valid if the stack is empty after iterating through the string.

Solution

Visualization


def isValid(s):
  stack = []
  mapping = {")": "(", "}": "{", "]": "["}
  for char in s:
    if char in mapping:
      if not stack or stack[-1] != mapping[char]:
        return False
      stack.pop()
    else:
      stack.append(char)
  return len(stack) == 0

valid parentheses

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