LeetCode 14. Longest Common Prefix 最长公共前缀

地址：https://leetcode.com/problems/longest-common-prefix/description/

题目：

 

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.

 

几种不错的解法：https://leetcode.com/articles/longest-common-prefix/

此处选取两种。

水平扫描：

    /**
     * 最长公共前缀 (水平扫描)
     * https://leetcode.com/problems/longest-common-prefix/description/
     * @param strs 原始字符串数组
     * @return 最长公共前缀
     */
    public String longestCommonPrefix(String[] strs) {
        if (strs.length == 0){ return "";}
        String prefix = strs[0];
        for (int i = 1; i < strs.length; i++)
        {
            while (strs[i].indexOf(prefix) != 0) {
                prefix = prefix.substring(0, prefix.length() - 1);
                if (prefix.isEmpty()) { return "";}
            }
        }
        return prefix;
    }

 

垂直扫描：

public String longestCommonPrefix(String[] strs) {
    if (strs == null || strs.length == 0) return "";
    for (int i = 0; i < strs[0].length() ; i++){
        char c = strs[0].charAt(i);
        for (int j = 1; j < strs.length; j ++) {
            if (i == strs[j].length() || strs[j].charAt(i) != c)
                return strs[0].substring(0, i);             
        }
    }
    return strs[0];
}

分治法

public String longestCommonPrefix(String[] strs) {
    if (strs == null || strs.length == 0) return "";    
        return longestCommonPrefix(strs, 0 , strs.length - 1);
}

private String longestCommonPrefix(String[] strs, int l, int r) {
    if (l == r) {
        return strs[l];
    }
    else {
        int mid = (l + r)/2;
        String lcpLeft =   longestCommonPrefix(strs, l , mid);
        String lcpRight =  longestCommonPrefix(strs, mid + 1,r);
        return commonPrefix(lcpLeft, lcpRight);
   }
}

String commonPrefix(String left,String right) {
    int min = Math.min(left.length(), right.length());       
    for (int i = 0; i < min; i++) {
        if ( left.charAt(i) != right.charAt(i) )
            return left.substring(0, i);
    }
    return left.substring(0, min);
}