Stacks: Balanced Brackets HackerRank Solution

Stacks: Balanced Brackets HackerRank Solution
Source : https://www.hackerrank.com/challenges/ctci-balanced-brackets

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, ].

Some examples of balanced brackets are []{}(), [({})]{}() and ({(){}[]})[].

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

YESNOYES

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.

Source : https://www.hackerrank.com/challenges/ctci-balanced-brackets

Solution

	// Karthikalapati.blogspot.com
	// ArrayDeque is "likely to be faster than Stack when used as a stack" - Java documentation
	// Time Complexity: O(n)
	// Space Complexity: O(n)
	/* Create HashMap to match opening brackets with closing brackets */
	static String isBalanced(String expression) {
	HashMap<Character, Character> map = new HashMap();
	map.put('(', ')');
	map.put('[', ']');
	map.put('{', '}');
	return isBalanced(expression, map) ? "YES" : "NO";
	}
	private static boolean isBalanced(String expression, HashMap<Character, Character> map) {
	if ((expression.length() % 2) != 0) {
	return false; // odd length Strings are not balanced
	}
	ArrayDeque<Character> deque = new ArrayDeque(); // use deque as a stack
	for (int i = 0; i < expression.length(); i++) {
	Character ch = expression.charAt(i);
	if (map.containsKey(ch)) {
	deque.push(ch);
	} else if (deque.isEmpty() || ch != map.get(deque.pop())) {
	return false;
	}
	}
	return deque.isEmpty();
	}