leetcode-20-valid-parentheses.swift

/**

Copyright (c) 2020 David Seek, LLC

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The above copyright notice and this permission notice shall be included in all
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
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OR OTHER DEALINGS IN THE SOFTWARE.





https://leetcode.com/problems/valid-parentheses/


Given a string 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.
Note that an empty string is also considered valid.




Example 1:

Input: "()"
Output: true
Example 2:

Input: "()[]{}"
Output: true
Example 3:

Input: "(]"
Output: false
Example 4:

Input: "([)]"
Output: false
Example 5:

Input: "{[]}"
Output: true

*/

/**
Big O Annotation
Time complexity O(n) where n is the amount of characters in s.
Space complexity O(n log n) as we're 
always at most holding 1/2 of characters in n
*/
func isValid(_ s: String) -> Bool {
	
	// Init a Character Stack
	var stack = Stack<Character>()
	
	// Iterate through given string
	for character in s {

		// Check if the character is (, { or [
		if isOpen(character) {
			
			// If so put into the stack
			stack.push(character)
			
		} else {
			
			// Pop the last element of the stack
			guard let element = stack.pop() else {
				return false 
			}
			
			// Check if it's the desired counter part
			if !isOpposite(character, of: element) {
				return false
			}
		}
	}
		
	// Check if the Stack is empty
	// If not then we did not close all brackets
	return stack.isEmpty 
}

func isOpen(_ character: Character) -> Bool {
	return character == "(" ||
	character == "{" ||
	character == "["
}

func isOpposite(_ character: Character, of stackElement: Character) -> Bool {
	if character == ")" {
		return stackElement == "("
	}
	
	if character == "}" {
		return stackElement == "{"
	}
	
	// character must be ]
	return stackElement == "["
}

struct Stack<Element: Equatable> {
	private var array: [Element]
	
	public var peek: Element? {
		return array.last
	}
	
	public var isEmpty: Bool {
		return array.isEmpty
	}
	
	init() {
		array = []		
	}
	
	mutating public func push(_ element: Element) {
		array.append(element)
	}
	
	mutating public func pop() -> Element? {
		return array.popLast()
	}
}