20. Valid Parentheses

Solution Code

Java

class Solution {
    public boolean isValid(String s) {
        Stack<Character> st = new Stack<Character>(); 
        
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(' || s.charAt(i) == '[' || s.charAt(i) == '{') {
                st.push(s.charAt(i));
            } else {
                char c;
                
            if (st.size() > 0) 
                c = st.pop();
            else 
                return false;

            if (s.charAt(i) == ']' && c != '[') 
                return false;

            if (s.charAt(i) == '}' && c != '{') 
                return false;

            if (s.charAt(i) == ')' && c != '(') 
                return false;
            }
        }
            
        if (st.size() > 0) 
            return false;

        return true;
    }
}