leetcode 705,为什么执行出错呢？

Question

 class MyHashSet {
public:
    /** Initialize your data structure here. */
    MyHashSet() {

    }

    void add(int key) {
        if (v[key] == 0)
            ++v[key];
    }

    void remove(int key) {
        if (v[key] > 0)
            --v[key];
    }

    /** Returns true if this set did not already contain the specified element */
    bool contains(int key) {
        if (v[key] > 0)
            return true;
        return false;
    }

private:

vector<int> v;    
};

/**
 * Your MyHashSet object will be instantiated and called as such:
 * MyHashSet obj = new MyHashSet();
 * obj.add(key);
 * obj.remove(key);
 * bool param_3 = obj.contains(key);
 */