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Grind 75.2 Valid Parentheses

Solved at: 2022-07-10

Question

Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

  1. Open brackets must be closed by the same type of brackets.
  2. Open brackets must be closed in the correct order.

Solution

This is a classic stack problem.

I missed some details:

  • Remember pop() doesn't take any argument.
  • Remember to check the final condition. When the list is not empty at the end, then return False.
class Solution:
def isValid(self, s: str) -> bool:
stack = []
opening = ["(", "{", "["]
closing = [")", "}", "]"]

for _, c in enumerate(s):
if c in opening:
stack.append(c)
continue
if not stack:
return False
e = stack.pop()
if opening.index(e) != closing.index(c):
return False
if not stack:
return True
return False

Results

Runtime

  • 67 ms, faster than 11.18% of Python3 online submissions for Valid Parentheses.

Memory Usage

  • 13.9 MB, less than 24.23% of Python3 online submissions for Valid Parentheses.

Other Answers Online

  • All of the other solutions I found had similar approaches.
  • This is funny.
public boolean isValid(String s) {
while (s.indexOf("{}") != -1 || s.indexOf("[]") != -1 ||
s.indexOf("()") != -1) {
s = s.replace("()", "");
s = s.replace("[]", "");
s = s.replace("{}", "");
}
return s.isEmpty();
}