20. Valid Parentheses

问题
Given a string s 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.

用栈储存遍历中的字符。
如果是“（”，“{”或“[”，则入栈。
如果是其他字符，且不与栈顶的字符成对，则返回false。
其他情况需要pop掉栈顶。

• toCharArray(): 将字符串转换为字符数组，便于遍历。

class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack();
        
        for (char c : s.toCharArray()){
            if ( c == '(' || c == '{' || c == '[' ){
                stack.push(c);
            }
            else if ( stack.size() == 0 || 
                     c == ')' && stack.peek() != '(' || 
                     c == '}' && stack.peek() != '{' || 
                     c == ']' && stack.peek() != '[') {
                return false;
            }
            else{
                stack.pop();
            }
        }
        return stack.isEmpty();
    }
}