20. Valid Parentheses

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Problem

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

Constraints:

• $1 <= s.length <= 10^4$
• s consists of parentheses only ‘()[]{}’.

Code

class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<>();
        for(char c : s.toCharArray()){
            if(c == ']' || c ==')' || c == '}') {
                if(stack.isEmpty() || stack.pop() != c) return false;
            } else {
                if(c == '[') {
                    stack.push(']');
                } else if (c == '{') {
                    stack.push('}');
                } else {
                    stack.push(')');
                }
            }
        }
        
        return stack.isEmpty();
    }
}