0020-valid-parentheses.java

class Solution {

    public boolean isValid(String s) {
        if (s.length() % 2 != 0) return false;
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            if (
                stack.isEmpty() &&
                (s.charAt(i) == ')' || s.charAt(i) == '}' || s.charAt(i) == ']')
            ) return false; else {
                if (!stack.isEmpty()) {
                    if (
                        stack.peek() == '(' && s.charAt(i) == ')'
                    ) stack.pop(); else if (
                        stack.peek() == '{' && s.charAt(i) == '}'
                    ) stack.pop(); else if (
                        stack.peek() == '[' && s.charAt(i) == ']'
                    ) stack.pop(); else stack.add(s.charAt(i));
                } else stack.add(s.charAt(i));
            }
        }
        return stack.isEmpty();
    }
}

//Solution with HashMap Lookup table as described in the video

class Solution {
    public boolean isValid(String s) {
        Stack<Character> brackets = new Stack<>();
        Map<Character, Character> bracketLookup = new HashMap<>(3);

        bracketLookup.put(')', '(');
        bracketLookup.put('}', '{');
        bracketLookup.put(']', '[');

        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (bracketLookup.containsKey(c)) {
                if (!brackets.isEmpty() && bracketLookup.get(c).equals(brackets.peek())) {
                    brackets.pop();
                } else {
                    return false;
                }
            } else {
                brackets.push(c);
            }
        }

        return brackets.isEmpty();
    }
}