Check for balanced parentheses in Python

Given an expression string, write a python program to find whether a given string has balanced parentheses or not.

Examples:

Input : {[]{()}}
Output : Balanced

Input : [{}{}(]
Output : Unbalanced

Approach #1: Using stack One approach to check balanced parentheses is to use stack. Each time, when an open parentheses is encountered push it in the stack, and when closed parenthesis is encountered, match it with the top of stack and pop it. If stack is empty at the end, return Balanced otherwise, Unbalanced. 

Python3

# Python3 code to Check for
# balanced parentheses in an expression
open_list = ["[","{","("]
close_list = ["]","}",")"]

# Function to check parentheses
def check(myStr):
    stack = []
    for i in myStr:
        if i in open_list:
            stack.append(i)
        elif i in close_list:
            pos = close_list.index(i)
            if ((len(stack) > 0) and
                (open_list[pos] == stack[len(stack)-1])):
                stack.pop()
            else:
                return "Unbalanced"
    if len(stack) == 0:
        return "Balanced"
    else:
        return "Unbalanced"


# Driver code
string = "{[]{()}}"
print(string,"-", check(string))

string = "[{}{})(]"
print(string,"-", check(string))

string = "((()"
print(string,"-",check(string))

Output:

{[]{()}} - Balanced
[{}{})(] - Unbalanced
((() - Unbalanced

Time Complexity: O(n), The time complexity of this algorithm is O(n), where n is the length of the string. This is because we are iterating through the string and performing constant time operations on the stack.
Auxiliary Space: O(n), The space complexity of this algorithm is O(n) as well, since we are storing the contents of the string in a stack, which can grow up to the size of the string.

Approach #2: Using queue First Map opening parentheses to respective closing parentheses. Iterate through the given expression using ‘i’, if ‘i’ is an open parentheses, append in queue, if ‘i’ is close parentheses, Check whether queue is empty or ‘i’ is the top element of queue, if yes, return “Unbalanced”, otherwise “Balanced”. 

Python3

# Python3 code to Check for
# balanced parentheses in an expression
def check(expression):

    open_tup = tuple('({[')
    close_tup = tuple(')}]')
    map = dict(zip(open_tup, close_tup))
    queue = []

    for i in expression:
        if i in open_tup:
            queue.append(map[i])
        elif i in close_tup:
            if not queue or i != queue.pop():
                return "Unbalanced"
    if not queue:
        return "Balanced"
    else:
        return "Unbalanced"

# Driver code
string = "{[]{()}}"
print(string, "-", check(string))

string = "((()"
print(string,"-",check(string))

Output:

{[]{()}} - Balanced
((() - Unbalanced

Time Complexity: O(n)
Auxiliary Space: O(n)

Approach#3: Elimination based In every iteration, the innermost brackets get eliminated (replaced with empty string). If we end up with an empty string, our initial one was balanced; otherwise, not. 

Python3

# Python3 code to Check for
# balanced parentheses in an expression
def check(my_string):
    brackets = ['()', '{}', '[]']
    while any(x in my_string for x in brackets):
        for br in brackets:
            my_string = my_string.replace(br, '')
    return not my_string

# Driver code
string = "{[]{()}}"
print(string, "-", "Balanced"
      if check(string) else "Unbalanced")

Output:

{[]{()}} - Balanced

Time Complexity: O(n)
Auxiliary Space: O(n)