20. 有效的括号

题目：

思路：

【1】无

代码展示：

//时间1 ms 击败 97.94%
//内存39.8 MB 击败 23.45%
class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<>();
        HashMap<Character, Character> check = new HashMap<>();
        check.put('(',')');
        check.put('{','}');
        check.put('[',']');
        for (char v : s.toCharArray()){
            if (v == '(' || v ==  '{' || v == '['){
                stack.add(v);
            }else {
                if (stack.size() > 0 && check.get(stack.pop()) == v){
                    continue;
                }else {
                    return false;
                }
            }
        }
        if (stack.size() > 0)
            return false;
        return true;
    }
}


//时间0 ms 击败 100%
//内存39.6 MB 击败 59.35%
class Solution {
    public boolean isValid(String s) {
        char[] stack = new char[s.length()];
        int top = -1;//栈顶指针
        for (char c : s.toCharArray()) {
            if (c == '(') {
                stack[++top] = ')';
            } else if (c == '[') {
                stack[++top] = ']';
            } else if (c == '{') {
                stack[++top] = '}';
            } else if (top == -1 || stack[top--] != c) {//之前没有对应的左括号或者左右括号不匹配
                return false;
            }
        }
        return top == -1;//判断是否有剩余的左括号没有被匹配
    }
}