# 14.Longest-Common-Prefix

## 14. Longest Common Prefix

## 题目地址

<https://leetcode.com/problems/longest-common-prefix/>

## 题目描述

```
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.
```

## 代码

### Approach #1 Horizontal scanning

```java
class Solution {
  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;
  }
}
```

### Approach #2 Divide and conquer

```java
class Solution {
  public String longestCommonPrefix(String[] strs) {
    if (strs == null || strs.length == 0)        return "";
    return lonestCommonPrefix(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);
  }

}
```

### Approach #3 Binary Search

```java
class Solution {
  public String longestCommonPrefix(String[] strs) {
    if (strs == null || strs.length == 0)        return "";
    int minLen = Integer.MAX_VALUE;
    for (String str: strs) {
      minLen = Math.min(minLen, str.length());
    }
    int low = l;
    int high = minLen;
    while (low <= high) {
      int middle = (low + high) / 2;
      if (isCommonPrefix(strs, middle)) {
        low = middle + 1;
      } else {
        high = middle - 1;
      }
    }

    return strs[0].substring(0, (low + high) / 2);
  }

  private boolean isCommonPrefix(String[] strs, int len) {
    String str1 = strs[0].substring(0, len);
    for (int i = 1; i < strs.length; i++) {
      if (!strs[i].startsWith(str1)) {
        return false;
      }
    }
    return true;
  }

}
```

### Approach #4 Vertical Scanning

```java
class Solution {
  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];
  }
}
```


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