14. Longest Common Prefix

最长公共前缀，遍历进行比较，注意indexOf和substring的范围

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

    String prefix = strs[0];
    for (int i = 0; i < strs.length; i++) {
        prefix = commonPrefix(prefix, strs[i]);
    }
    return prefix;
}

public String commonPrefix(String prefix, String str) {
    while (str.indexOf(prefix) != 0) { 
        prefix = prefix.substring(0, prefix.length() - 1);
    }
    return prefix;
}