LC20 有效的括号

struct Stack {
    char *val;
    int top, maxSize;
};

void init(struct Stack *s, int maxSize) {
    s->val = malloc(maxSize);
    s->top = 0;
}

bool empty(struct Stack *s) {
    return !(s->top);
}

void push(struct Stack *s, char c) {
    s->val[s->top++] = c;
}

void pop(struct Stack *s) {
    s->top--;
}

char seek(struct Stack *s) {
    return s->val[s->top - 1];
}

bool isValid(char *s) {
    int length = strlen(s);
    struct Stack stack;
    init(&stack, length);
    for (int i = 0; i < length; i++) {
        switch(s[i]) {
            case '(':
            case '{':
            case '[':
                   push(&stack, s[i]);
                   break;
            case ')':
                    if (empty(&stack)) return false;
                    if (seek(&stack) != '(') return false;
                    pop(&stack);
                    break;
            case '}':
                    if (empty(&stack)) return false;
                    if (seek(&stack) != '{') return false;
                    pop(&stack);
                    break;
            case ']':
                    if (empty(&stack)) return false;
                    if (seek(&stack) != '[') return false;
                    pop(&stack);
                    break;
        }
    }
    return empty(&stack);
}