Hackerrank - Data Structure - Stacks - Balanced Bracket

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 considered to be 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, print YES on a new line; otherwise, print NO on a new line.

Input Format

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

Constraints

• , where  is the length of the sequence.
• Each character in the sequence will be a bracket (i.e., {, }, (, ), [, and ]).

Output Format

For each string, print whether or not the string of brackets is balanced on a new line. If the brackets are balanced, print YES; otherwise, print 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 pairs [(] and (]) are not balanced. Thus, we print NO on a new line.
3. The string {{[[(())]]}} meets both criteria for being a balanced string, so we print YES on a new line,

Solution:

def is_matched(expression):
    stack = []
    mapper = {'{': '}', '(':')', '[': ']'}
    for u in expression:
        if u in mapper:
            stack.append(u)
        elif not stack or mapper[stack[-1]] != u:
            return False
        else:
            stack.pop()
    return not len(stack)

t = int(raw_input().strip())
for a0 in xrange(t):
    expression = raw_input().strip()
    if is_matched(expression) == True:
        print "YES"
    else:
        print "NO"