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

這個題是簡單的字串匹配

1）

class Solution {
    public boolean isValid(String s) {
        if(s == null || s.length() < 1){
            return true;
        }else if(s.length() == 1){
            return false;
        }
        Stack<Character> stack = new Stack<>();
        for(int i = 0 ; i < s.length();i++){
            if(s.charAt(i) != ')' && s.charAt(i) != ']' && s.charAt(i) != '}'){
                stack.push(s.charAt(i));
            }else{
                if(!stack.isEmpty() && stack.peek() == '(' && s.charAt(i) == ')'){
                    stack.pop();
                }else if(!stack.isEmpty() && stack.peek() == '[' && s.charAt(i) == ']'){
                    stack.pop();
                }else if(!stack.isEmpty() && stack.peek() == '{' && s.charAt(i) == '}'){
                    stack.pop();
                }else{
                    return false;
                }
            }
        }
        if(!stack.isEmpty()){
            return false;
        }else{
            return true;
        }
    }
}
 

2）

class Solution {
    public boolean isValid(String s) {
        int top = -1;
        char[] cs = s.toCharArray();
        for(int i=0;i<s.length();i++){
            if(top < 0 || !ism(cs[top], cs[i])){
                top++;
                cs[top] = cs[i];// 如果只有一種括號就不需要這行
            }
            else
                top--;
        }
        return top == -1;
    }
    
    public boolean ism(char c1, char c2){
        if(c1 == '(' && c2 ==')')
            return true;
        if(c1 == '[' && c2 ==']')
            return true;
        if(c1 == '{' && c2 =='}')
            return true;
        return false;
    }
}