Longest Common Prefix

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

Steps:

1. Corner check
2. Assign strs[0] to String prefix variable
3. Get the common prefix of strs[i] and prefix
   1. Be careful that prefix may be longer than str[i], so need to cut the prefix off if it is longer than strs[i] before element-by-element comparison (otherwise it is easy to have null pointer exception)

// Simplified code

class Solution {

    public String longestCommonPrefix(String[] strs) {
        if (strs == null) {
            return null;
        }

        if (strs.length == 0) {
            return "";
        }

        if (strs.length == 1) {
            return strs[0];
        }

        String prefix = strs[0];

        for (int i = 1; i < strs.length; i++) {

            int j = 0;

            while (j < prefix.length() && j < strs[i].length() &&
                   prefix.charAt(j) == strs[i].charAt(j)) {
                j++;
            }

            if (j == 0) {
                return "";
            }

            prefix = prefix.substring(0, j);
        }

        return prefix;
    }
}

// My own code, can be simplified, use while loop so that you can have multiple conditions

class Solution {

    public String longestCommonPrefix(String[] strs) {
        if (strs == null) {
            return null;
        }

        if (strs.length == 0) {
            return "";
        }

        if (strs.length == 1) {
            return strs[0];
        }

        String prefix = strs[0];

        for (int i = 1; i < strs.length; i++) {
            prefix = getCommonPrefix(prefix, strs[i]);
            if (prefix.equals("")) {
                break;
            }
        }

        return prefix;
    }

    public String getCommonPrefix(String prefix, String str) { 

        // prefix may be longer than str

        prefix = (prefix.length() > str.length()) ? prefix.substring(0, str.length()) : prefix;

        for (int i = 0; i < prefix.length(); i++) {
            if (prefix.charAt(i) != str.charAt(i)) {
                prefix = str.substring(0, i);
                break;
            }
        }
        return prefix;
    }
}