Hackerrank - Balanced Brackets Solution

A bracket is considered to be any one of the following characters: (, ), {, }, [, or ].

Two brackets are considered to be a matched pair if the an opening bracket (i.e., (, [, or {) occurs to the left of a closing bracket (i.e., ), ], or }) of the exact same type. There are three types of matched pairs of brackets: [], {}, and ().

A matching pair of brackets is not balanced if the set of brackets it encloses are not matched. For example, {[(])} is not balanced because the contents in between { and } are not balanced. The pair of square brackets encloses a single, unbalanced opening bracket, (, and the pair of parentheses encloses a single, unbalanced closing square bracket, ].

By this logic, we say a sequence of brackets is balanced if the following conditions are met:

• It contains no unmatched brackets.
• The subset of brackets enclosed within the confines of a matched pair of brackets is also a matched pair of brackets.

Given  strings of brackets, determine whether each sequence of brackets is balanced. If a string is balanced, return YES. Otherwise, return NO.

Function Description

Complete the function isBalanced in the editor below. It must return a string: YES if the sequence is balanced or NO if it is not.

isBalanced has the following parameter(s):

• s: a string of brackets

Input Format

The first line contains a single integer , the number of strings.
Each of the next  lines contains a single string , a sequence of brackets.

Constraints

• , where  is the length of the sequence.
• All chracters in the sequences ∈ { {, }, (, ), [, ] }.

Output Format

For each string, return YES or NO.

Sample Input

3
{[()]}
{[(])}
{{[[(())]]}}

Sample Output

YES
NO
YES

Explanation

1. The string {[()]} meets both criteria for being a balanced string, so we print YES on a new line.
2. The string {[(])} is not balanced because the brackets enclosed by the matched pair { and } are not balanced: [(]).
3. The string {{[[(())]]}} meets both criteria for being a balanced string, so we print YES on a new line.

Solution in Python using stack

def isBalanced(S):
    stack = []
    pairs = {"{": "}", "[": "]", "(" : ")"}
    for i in S:
        if not stack:
            stack.append(i)
        elif i == pairs.get(stack[-1]):
            stack.pop()
        else:
            stack.append(i)
    return "YES" if not stack else "NO"


for i in range(int(input())):
    S = input()
    print(isBalanced(S))

Solution in Python using regex

Not recommended

import re

def isBalanced(s):
    p= "\[\]|\(\)|\{\}"
    while re.search(p,s):
        s = re.sub(p,"",s)
    return "NO" if s else "YES"

for i in range(int(input())):
    print(isBalanced(input()))

Explanation

Basically we are continually using regex to search for (), {} or [] and replace it with an empty string and break the loop once no more such match is found.

If we successfully replace all matches and the resultant value of s is empty string then our function will return "YES".

Example

Loop 1 >>> s = "{[()]}"
Loop 2 >>> s = "{[]}"
Loop 3 >>> s = "{}"
Loop 4 >>> s = ""

If we have a unbalanced parenthesis such as {] then our program will not be able to replace all values of the given string and our function will return "NO".

Loop 1 >>> s = "[[()]{(]}]"
Loop 2 >>> s = "[[]{(]}]"
Loop 3 >>> s = "[{(]}]"