20. Valid Parentheses

用stack

 

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