Valid Parentheses

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.

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这道题目比较简单，用堆栈来处理，当遇到‘(’, ‘{’, ‘[’ 时就push到堆栈中，当遇到‘)’, ‘}’, ‘]’时，首先判断堆栈是否为空，如果为空说明之前字符中没有与right braces匹配的，返回false；如果堆栈不为空，判断是否匹配，如果匹配就继续往下匹配，如果不匹配直接返回false；最后要检查堆栈是否为空。代码如下：

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