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

方法：从开始依次比较

思路：
选一个one_str作为参照，对其他strs都按位与one_str比较，直到出界或不相等
Time Complexity: O(n*min(k)) n个str,长度为k
Space Complexity: O(1)

Solution Code：

class Solution {
    public String longestCommonPrefix(String[] strs) {
        if(strs == null || strs.length == 0) return "";
        
        String one_str = strs[0];    // as reference
        int max_length = 0;
        // compare all strs with one_str char by char
        for(int pos = 0; pos < one_str.length(); pos++) {
            for(int i = 1; i < strs.length; i++) {
                if(pos >= strs[i].length() || strs[i].charAt(pos) != one_str.charAt(pos)) 
                {
                    return one_str.substring(0, max_length);
                }
            }
            max_length++;
        }
        return one_str.substring(0, max_length);
    }
}

class Solution {
    public String longestCommonPrefix(String[] strs) {
        if(strs == null || strs.length == 0) {
            return "";
        }
        
        String pre = strs[0];
        int i = 1;
        
        while(i < strs.length) {
            while(strs[i].indexOf(pre) != 0) {
                pre = pre.substring(0, pre.length() - 1);
            }
            i++;
        }
        
        return pre;
    }
}

class Solution {
    public String longestCommonPrefix(String[] strs) {
        if(strs == null || strs.length == 0 || strs[0].length() == 0) return "";
        int i = 0;
        while(true) {
            if(i >= strs[0].length()) return strs[0].substring(0, i);
            Character c = strs[0].charAt(i); 
            for(String str: strs) {
                if(i >= str.length() || str.charAt(i) != c) {
                    return strs[0].substring(0, i);
                }
            }
            i++;
        }
        
    }
}