Valid Parentheses

Parentheses.java

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

// An input string is valid if:

// Open brackets must be closed by the same type of brackets.
// 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 Parentheses {
  
    public static void main(String args[]) {
        if(isValid("()")) {
          System.out.println("Valid");
        } else {
          System.out.println("Not Valid");
        }
    }
  
    public static boolean isValid(String s) {
        Stack<Character> stack = new Stack<Character>();
        for(int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
             if (c == '{' || c == '(' || c == '[') {
                 stack.push(c);
             }
             if (c == '}' || c == ')' || c == ']') {
                if(stack.empty()) { 
                   return false; 
                }
                else if (!isMatchingPair(stack.pop(), c) ) 
               { 
                   return false; 
               } 
          }
        }
        return stack.empty();
    }
    
    private static boolean isMatchingPair(char character1, char character2) 
    { 
        return (character1 == '(' && character2 == ')') || (character1 == '{' && character2 == '}') || 
                (character1 == '[' && character2 == ']');
    }
}