CODE 111. Valid Parentheses

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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.

public boolean isValid(String s) {
	// IMPORTANT: Please reset any member data you declared, as
	// the same Solution instance will be reused for each test case.
	char[] ss = s.toCharArray();
	Stack<Character> stack = new Stack<Character>();
	for (char c : ss) {
		if ('(' == c || '{' == c || '[' == c) {
			stack.add(c);
		} else {
			if (stack.isEmpty()) {
				return false;
			}
			if (')' == c && stack.pop() != '(') {
				return false;
			} else if (']' == c && stack.pop() != '[') {
				return false;
			} else if ('}' == c && stack.pop() != '{') {
				return false;
			}
		}
	}
	if (stack.isEmpty()) {
		return true;
	} else {
		return false;
	}
}