class Solution {
    public String longestCommonPrefix(String[] strs) {
        String pre = strs[0];
        int index = 0, len = pre.length();
        for (int i=1; i<strs.length; i++) {
            len = Math.min(len, strs[i].length());
            while (index < len) {
                if (pre.charAt(index) != strs[i].charAt(index)) break;
                index++;
            }
            if (index == 0) return "";
            pre = pre.substring(0, index);
            len = index;
            index = 0;
        }
        return pre;
    }
}