20. Valid Parentheses (JAVA)

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<>(); //Primitive types such as char cannot be used as type parameters in Java. You need to use the wrapper type

        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) == ')') {
                if(stack.empty() || stack.peek() != '(') return false;
                stack.pop();
            }
            else if(s.charAt(i) == ']'){
                if(stack.empty() || stack.peek() != '[') return false;
                stack.pop();
            }
            else if(s.charAt(i) == '}'){
                if(stack.empty() || stack.peek() != '{') return false;
                stack.pop();
            }
        }
        if(!stack.empty()) return false;
        return true;
    }
}