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1：经典的栈匹配。一个栈，左符号入栈，右符号出栈。最后检查栈是否为空。

public class Solution {	public boolean isValid(String s) {		if (s == null || s.length() == 0)			return true;		Stack
    
      stack = new Stack
     
      ();		int n = s.length();		for (int i = 0; i < n; i++) {			char ch = s.charAt(i);			if (ch == '(' || ch == '{' || ch == '[')				stack.push(ch);			else {				if (stack.isEmpty())					return false;				char out = stack.pop();				if (ch == ')' && out != '(' || ch == '}' && out != '{'						|| ch == ']' && out != '[')					return false;			}		}		if (!stack.isEmpty())			return false;		return true;	}	public static void main(String[] args) {		// true		System.out.println(new Solution().isValid("()"));		// true		System.out.println(new Solution().isValid("()[]{}"));		// false		System.out.println(new Solution().isValid("(]"));		// false		System.out.println(new Solution().isValid("([)]"));	}}