Valid Parentheses

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Problem

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.

3. Every close bracket has a corresponding open bracket of the same type.

Pseudocode

usage of opening and closing parenthesis follows last in first out (LIFO), the last opening parenthesis must follow with the first corresponding closing parenthesis.

- create a map that has a [opening parenthesis, closing parenthesis], key-value pair
- create an empty array as a stack
- loop through the string
    - if character is an opening parenthesis
        - push corresponding closing parenthesis into stack
    - if character is a closing parenthesis
        - pop last element from stack and compare if it is the right pair
        - if it's not, return false immediately
- when the loop completes and
    - if the stack is empty, return true
    - else, return false

Solution

var isValid = function (s) {
  const closingTagsMap = {
    "(": ")",
    "{": "}",
    "[": "]",
  };
  let stack = [];
  let len = s.length;
  if (len % 2 !== 0) {
    return false;
  }

  for (let i = 0; i < len; i++) {
    if (closingTagsMap[s[i]]) {
      stack.push(s[i]);
    } else {
      if (s[i] !== closingTagsMap[stack.pop()]) {
        return false;
      }
    }
  }

  return !stack.length;
};

Time and Space Complexity

Time

• Loops through string once to find opening and closing parenthesis

• Lookup for corresponding parenthesis using Map is constant time

• Total - O(N)

Space

• Space complexity increases linearly with input i.e. stack length

• Total - O(N)