LeetCode_20. Valid Parentheses

package leetcode.easy;

import java.util.HashMap;
import java.util.Stack;

public class ValidParentheses {

	// Hash table that takes care of the mappings.
	private HashMap<Character, Character> mappings;

	// Initialize hash map with mappings. This simply makes the code easier to
	// read.
	public ValidParentheses() {
		mappings = new HashMap<Character, Character>();
		mappings.put(')', '(');
		mappings.put('}', '{');
		mappings.put(']', '[');
	}

	public boolean isValid(String s) {

		// Initialize a stack to be used in the algorithm.
		Stack<Character> stack = new Stack<Character>();
		for (int i = 0; i < s.length(); i++) {
			char c = s.charAt(i);

			// If the current character is a closing bracket.
			if (this.mappings.containsKey(c)) {

				// Get the top element of the stack. If the stack is empty, set
				// a dummy value of '#'
				char topElement = stack.isEmpty() ? '#' : stack.pop();

				// If the mapping for this bracket doesn't match the stack's top
				// element, return false.
				if (topElement != this.mappings.get(c)) {
					return false;
				}
			} else {
				// If it was an opening bracket, push to the stack.
				stack.push(c);
			}
		}

		// If the stack still contains elements, then it is an invalid
		// expression.
		return stack.isEmpty();
	}

	@org.junit.Test
	public void test() {
		String s1 = "()";
		String s2 = "()[]{}";
		String s3 = "(]";
		String s4 = "([)]";
		String s5 = "{[]}";

		System.out.println(isValid(s1));
		System.out.println(isValid(s2));
		System.out.println(isValid(s3));
		System.out.println(isValid(s4));
		System.out.println(isValid(s5));
	}
}