3-1

class Solution {
    public String longestCommonPrefix(String[] strs) {
        if (strs == null || strs.length == 0) {
            return "";
        }
        // Initialize 'prefix' with the longest possible prefix.
        // 'prefix' cannot be longer than any of the strings, so
        // simply initialize it with strs[0].
        String prefix = strs[0];

        // Iterate over all strings.
        for (int i = 1; i < strs.length; i++) {
            // Until 'prefix' is not a prefix of the current string,
            // reduce 'prefix' by one character and try to match again.
            while (strs[i].indexOf(prefix) != 0) {
                // Removing the last character from 'prefix'.
                prefix = prefix.substring(0, prefix.length() - 1);
            }
        }
        // Return found 'prefix' as it is a prefix of all strings in the list.
        return prefix;
    }
}