20. Valid brackets

20. Valid parenthesis
    Given one only includes ‘(’,’)’,’{’,’}’,’[’,’]’ String s , Determines whether the string is valid .

Valid string needs to meet ：

Opening parentheses must be closed with closing parentheses of the same type .
The left parenthesis must be closed in the correct order .

analysis ：
This problem looks quite simple , Just use stack to realize . Who ever thought that one would , Do it wrong .
1、stack Methods ：
Push ：push(Object o)
Out of the stack ：pop()
Get stack top element ：peek(）
Sentenced to empty ：empty(） No isEmpty()

2、String Methods
Traverse ：
char s.charAt();
String s.substring(i,i+1);
substring It's all lowercase
The first parameter , The starting position , from 0 rise ; The second parameter , Termination position , It doesn't contain .

3、 Judge . Put left parentheses in the stack , A right bracket matches the left bracket at the top of the stack , Then the element at the top of the stack will be released . otherwise , Traverse element stack .