Leetcode14.寻找最长公共前缀为什么输入空string就报错呢

```class Solution {
public:
    string longestCommonPrefix(vector<string>& strs) {
    int num = 0;
    int check = 0;
    string Prefix;
    while(num <= strs[0].size()){
        for (int i = 1; i < strs.size(); ++i) {
            if (strs[0][num] == strs[i][num]){
                check ++;
            }
        }
        if (check == strs.size()-1){
            Prefix +=strs[0][num];
        }else{
            return Prefix;
        }
        num++;
        check = 0;
    }
    return Prefix;
    }
};

```