leetcode -- Valid Parentheses

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

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

思路：使用栈，java Stack类中 peek方法是查看栈顶元素而不移除它

public class Solution {
    public boolean isValid(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        Stack<Character> stack = new Stack<Character>();
        int len = s.length();
        boolean flag = false;
        for(int i = 0; i < len; i++){
            if(!stack.empty()){
                char top = stack.peek();
                char c = s.charAt(i);
     
                if(!match(top, c))
                    stack.push(c);
                else{
                    stack.pop();
                }
            } else {
                char c = s.charAt(i);
                stack.push(c);
            }
        }
        
        if(!stack.empty())
            return flag;
        else{
            flag = true;
            return flag;
        }
    }
    
    boolean match(char a, char b){
        if(a == '(' && b == ')'){
            return true;
        } else if(a == '{' && b == '}'){
            return true;
        } else if(a == '[' && b == ']'){
            return true;
        } else {
            return false;
        }
    }
}