20. 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.

验证一个字符串里的括号是否正确闭合 比较简单 代码如下

public boolean isValid(String s) {
    Stack<Character> stack = new Stack<Character>();
    for(char c : s.toCharArray()){
        switch (c){
            case '(':
            case '{':
            case '[':
                stack.push(c);
                break;
            case ')':
                if(stack.isEmpty() || stack.pop() != '(') return false;
                break;
            case '}':
                if(stack.isEmpty() || stack.pop() != '{') return false;
                break;
            case ']':
                if(stack.isEmpty() || stack.pop() != '[') return false;
                break;
        }
    }
    return stack.isEmpty();
}

代码已经很简洁了 但是没想到还有更骚的

public boolean isValid(String s) {
    Stack<Character> stack = new Stack<Character>();
    for (char c : s.toCharArray()) {
        if (c == '(')
            stack.push(')');
        else if (c == '{')
            stack.push('}');
        else if (c == '[')
            stack.push(']');
        else if (stack.isEmpty() || stack.pop() != c)
            return false;
    }
    return stack.isEmpty();
}

每次遇到左括号就向stack中放入右括号 遇到右括号直接和栈顶比较是否相等