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Interview Question Valid Parentheses

Last update: 1/6/2017 8:58:00 AM

By Fatih KABAKCI

The problem is to find out if the input are composed by valid parentheses. The opening and closing brackets must be closed in the correct way.

Problem #20 - Valid Parentheses

Description: Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

Difficulty: Easy

Solution in Java

package com.fatihkabakci.Easy.ValidParentheses;

import java.util.LinkedList;

/**
 * @author fkabakci
 * 
 * Problem Description: Given a string containing just the characters "(", ")", "{", "}", "[" and "]", 
 * determine if the input string is valid. The brackets must close in the correct order, 
 * "()" and "()[]{}" are all valid but "(]" and "([)]" are not.
 * 
 * Solution:
 * 
 * Go to first closing bracket, check whether the previous one is the correct opening tag. If so, remove those characters from the list,
 * until the list is empty.
 * 
 * 1. Put all characters to a collection list.
 * 2. While i : 0 to size of list
 * 	  if the character is one of the closing brackets - ")", "}", "]"
 * 		 a. Find out the conjugate one - "(", "{", "["
 * 		 b. Get the just previous character of current i in the list.
 * 		 c. Check them out if they are equal
 * 		 d. Remove both characters from the list.
 * 3. At the end, list should be empty if the matches are valid.
 */

public class ValidParentheses {
	public char conjugate(char c) {
		if (c == ")")
			return "(";
		else if (c == "]")
			return "[";
		else
			return "{";
	}

	public boolean isValid(String s) {
		LinkedList<Character> o = new LinkedList<Character>();
		for (char bracket : s.toCharArray()) {
			o.add(bracket);
		}

		int i = 0;
		while (i < o.size()) {
			char c = o.get(i);
			if (c == ")" || c == "}" || c == "]") {
				char _c = conjugate(c);
				char pc = (i > 0) ? o.get(i - 1) : "\0";
				if (pc != _c)
					return false;
				o.remove(i);
				o.remove(i - 1);
				i = i - 2;
			}
			i++;
		}
		return o.isEmpty();
	}

	public static void main(String[] args) {
		ValidParentheses v = new ValidParentheses();
		System.out.println(v.isValid("]"));
	}
}

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