Valid Parentheses

错误：注意在循环内检查stack是否为空

Given a string 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.

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

public boolean isValidParentheses(String s) {
        // Write your code here
        Stack<Character> stack = new Stack<>();
        for (char c : s.toCharArray()) {
            if (c == '(' || c == '[' || c == '{') {
                stack.push(c);
            }
            if (c == ')') {
                if (stack.isEmpty() || stack.pop() != '(') {
                    return false;
                }
            }
            if (c == ']') {
                if (stack.isEmpty() || stack.pop() != '[') {
                    return false;
                }
            }
            if (c == '}') {
                if (stack.isEmpty() || stack.pop() != '{') {
                    return false;
                }
            }
        }
        return stack.isEmpty();
    }