编写一个函数来查找字符串数组中的最长公共前缀。
    如果不存在公共前缀，返回空字符串 ""。
    示例 1:

    1. 输入: ["flower","flow","flight"]
    2. 输出: "fl"

    示例 2:

    1. 输入: ["dog","racecar","car"]
    2. 输出: ""
    3. 解释: 输入不存在公共前缀。

    解法1：逐个比较

    1. var longestCommonPrefix = function (strs) {
    2.     if (strs == null || strs.length === 0) return "";
    4.     let first = strs[0];
    6.     for (let i = 1; i < strs.length; i++) {
    7.         let j = 0;
    8.         for (; j < first.length && j < strs[i].length; j++) {
    9.             if (first[j] !== strs[i][j]) break;
    10.         }
    12.         first = first.substring(0, j);
    14.         if (!first) return "";
    15.     }
    17.     return first;
    18. }

    解法2：比较最大字符串和最小字符串

    1. var longestCommonPrefix = function (strs) {
    2.     if (strs == null || strs.length === 0) return "";
    4.     let max = strs[0];
    5.     let min = strs[0];
    6.     for (let i = 1; i < strs.length; i++) {
    7.        max = strs[i] > max ? strs[i] : max;
    8.        min = strs[i] > min ? min : strs[i];
    9.     }
    11.     let i = 0;
    12.     for (; i < min.length; i++) {
    13.        if(min[i]!==max[i]) break;
    14.     }
    15.     return min.substring(0,i);
    16. }

    解法3：分治思想
    将求N个字符串最长公共前缀的问题，拆解为求2个字符串最长公共前缀的子问题，最后归并2个字符串求值结果。

    1. var longestCommonPrefix = function (strs) {
    2.     if (strs == null || strs.length === 0) return "";
    4.     return longestPrefix(strs);
    5. }
    7. function longestPrefix(strs){
    8.   if(strs.length === 1) return strs[0];
    10.   const middle = Math.floor(strs.length/2);
    11.   const left = strs.slice(0, middle);
    12.   const right = strs.slice(middle);
    14.   return longestPrefixTwo(longestPrefix(left),longestPrefix(right));
    15. }
    17. function longestPrefixTwo(str1, str2){
    18.     let i = 0;
    19.     for (; i < str1.length && i < str2.length; i++) {
    20.        if(str1[i]!==str2[i]) break;
    21.     }
    22.     return str2.substring(0,i);
    23.  }