[LeetCode][easy]Longest Commong Prefix

分治法，l, mid, r 分爲 (l, mid) 和 (mid+1, r)

class Solution {
    public String longestCommonPrefix(String[] strs) {
        String result = "";
        if (strs == null || strs.length == 0) {
            return result;
        }
        return prefix(strs, 0, strs.length - 1);
    }
    private String prefix(String[] strs, int left, int right) {
        if (strs == null || strs.length == 0) {
            return "";
        }
        if (left == right) {
            return strs[left];
        }
        int mid = left + (right - left) / 2;
        return commonPrefix(prefix(strs, left, mid), prefix(strs, mid+1, right));
    }
    private String commonPrefix(String left, String right) {
        if (left == null || left.length() == 0 || right == null || right.length() == 0) {
            return "";
        }
        int minLength = left.length() < right.length() ? left.length() : right.length();
        for (int i=0; i<minLength; i++) {
            if (left.charAt(i) != right.charAt(i)) {
                if (i == 0) {
                    return "";
                }
                return left.substring(0, i);
            }
        }
        return left.substring(0, minLength);
    }
}