1. Horizontal

class Solution {
    public String longestCommonPrefix(String[] strs) {
        if (strs == null || strs.length == 0) return "";
        String prefix = strs[0];

        for (int i = 1; i < strs.length; i++){
            while (strs[i].indexOf(prefix) != 0){
                prefix = prefix.substring(0, prefix.length() - 1);
                if (prefix == null || prefix.length() == 0) return "";
            }
        }

        return prefix;
    }
}

1. Vertical

class Solution {
    public String longestCommonPrefix(String[] strs) {
        if (strs == null || strs.length == 0) return "";

        for (int i = 0; i < strs[0].length(); i++){
            char c = strs[0].charAt(i);
            for (int j = 1; j < strs.length; j++){
                if (i == strs[j].length() || strs[j].charAt(i) != c) return strs[0].substring(0, i);
            }
        }

        return strs[0];
    }
}

1. divide and concur

class Solution {
    public String longestCommonPrefix(String[] strs) {
        if (strs == null || strs.length == 0) return "";

        return divide(strs, 0, strs.length - 1);
    }

    private String divide(String[] strs, int left, int right){
        if (left == right){
            return strs[left];
        }

        int mid = left + (right - left) / 2;
        String leftPart = divide(strs, left, mid);
        String rightPart = divide(strs, mid + 1, right);

        return combine(leftPart, rightPart);
    }

    private String combine(String str1, String str2){
        int min = Math.min(str1.length(), str2.length());

        for (int i = 0; i < min; i++){
            if (str1.charAt(i) != str2.charAt(i)){
                return str1.substring(0, i);
            }
        }

        return str1.substring(0, min);
    }
}

Further Thoughts / Follow up

https://leetcode.com/problems/longest-common-prefix/solution/