Write a function to find the longest common prefix string amongst an array of strings.

没啥难度，不多说了

    public String longestCommonPrefix(String[] strs) {
        if(strs.length < 1) return "";
        String pre = strs[0];
        int i =0;
        while(i<strs.length){
            while(strs[i].indexOf(pre) != 0){
                pre = pre.substring(0,pre.length() -1);
            }
            i++;
        }
        return pre;
    }