14. Longest Common Prefix 最长公共前缀

题目：Roman to Integer 罗马字符转整数

难度：简单

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

If there is no common prefix, return an empty string "".

Example 1:

Input: ["flower","flow","flight"]
Output: "fl"

Example 2:

Input: ["dog","racecar","car"]
Output: ""
Explanation: There is no common prefix among the input strings.

Note:

All given inputs are in lowercase letters a-z.

题意解析：

写一个方法从一个字符串数组中查找最长的公共前缀，如果没有，返回""

解题思路一：

依次遍历每一个数组项，记录其公共的字符串，最后返回

public String longestCommonPrefix(String[] strs) {
       StringBuilder sb = new StringBuilder();

        if (strs == null || strs.length == 0){
            return "";
        }
        if (strs[0].length() == 0){
            return "";
        }else {
            for (int i = 0; i < strs[0].length(); i++) {
                char temp = strs[0].charAt(i);
                int j = 0;
                while (j < strs.length){
                    if (i+1 > strs[j].length()){
                        return sb.toString();
                    }
                    if (strs[j].equals("")){
                        return "";
                    }
                    if (strs[j].charAt(i) == temp){
                        j++;
                    }else {
                        return sb.toString();
                    }
                }
                sb.append(temp); 
            }
        }
        return sb.toString(); 
    }

此算法时间复杂度O(n²)

提交代码之后：

Runtime: 1 ms, faster than 94.61% of Java online submissions for Longest Common Prefix.

Memory Usage: 39.1 MB, less than 10.90% of Java online submissions for Longest Common Prefix.

解题思路二：

直接取第一个的长度假定为最长的长度，然后依次取数组项中，匹配是否包含这个字符串，如果匹配，直接返回，否则将第一项的长度前移一位，继续匹配。

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;
}

此算法时间复杂度为

提交代码之后：

Runtime: 0 ms, faster than 100.00% of Java online submissions for Longest Common Prefix.

Memory Usage: 39 MB, less than 15.33% of Java online submissions for Longest Common Prefix.