Longest Common Prefix

#algorithms #csharp #leetcode #easy

#14.Longest Common Prefix

Problem statement

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: strs = ["flower","flow","flight"]
Output: "fl"

Example 2

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

Explanation

找出陣列中所有元素最長的相同字符並返回，如果沒有相同字符則返回空字串 ""。

例如 cat 和 carry 相同的前綴字符為 ca

Solution

用雙迴圈逐一比對，第一層遍歷元素長度，第二層則遍歷陣列內所有元素。

但是找尋陣列內元素時，不可使用超出元素長度的索引，故先找到陣列內最短長度的元素，再以此長度作為第一層迴圈的中止值。

public string LongestCommonPrefix(string[] strs)
{
    if (strs.Length == 0) return null;

    string res = string.Empty;
    string str = strs[0];

    foreach (var s in strs)
    {
        if (s.Length < str.Length)
            str = s;
    }

    int len = strs.Length;

    for (int i = 0; i < str.Length; i++)
    {
        for (int j = 0; j < len; j++)
        {
            if (str[i] == strs[j][i])
                continue;
            else
                return res;
        }

        res += str[i];
    }

    return res;
}