ValidParentheses.java

// 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 class ValidParentheses {
    public boolean isValid(String s) {
        if(s.length() % 2 == 1) {
            return false;
        }
        
        Stack<Character> stack = new Stack<Character>();
        
        for(int i = 0; i < s.length(); i++) {
            if(s.charAt(i) == '(' || s.charAt(i) == '[' || s.charAt(i) == '{') {
                stack.push(s.charAt(i));
            } else if(s.charAt(i) == ')' && !stack.isEmpty() && stack.peek() == ')') {
                stack.pop();
            } else if(s.charAt(i) == ']' && !stack.isEmpty() && stack.peek() == ']') {
                stack.pop();
            } else if(s.charAt(i) == '}' && !stack.isEmpty() && stack.peek() == '}') {
                stack.pop();
            } else {
                return false;
            }
        }
        
        return stack.isEmpty();
    }
}