Valid Parentheses

原题：

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.

分析：

简单的括号匹配问题，用栈解决

注意边界条件

public boolean isValid(String s) {Stack<Character> stack = new Stack<Character>();for(int i=0; i<s.length(); i++) {char c = s.charAt(i);if("({[".contains(String.valueOf(c))) {stack.push(c);} else {if(!stack.isEmpty() && charMatch(stack.peek(), c)) {stack.pop();} else {return false;}}}return stack.isEmpty();    }private boolean charMatch(char c1, char c2) {if(c1=='(' && c2==')')return true;if(c1=='{' && c2=='}')return true;if(c1=='[' && c2==']')return true;return false;}