# 14. Longest Common Prefix

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`

## algorithm thought

以第一个字符串为基准，从第一个字符开始，对比所有字符串。出现不一样的就返回，都是一样的就加入到返回值后面。

## code

```
class Solution {
public:
    string longestCommonPrefix(vector<string>& strs) {
        if(strs.size()==1)
            return strs[0];
        if(strs.size()==0)
            return "";
        string res="";
        for(int i=0;i<strs[0].size();++i){
            for(int j=1;j<strs.size();++j){
                if(i==strs[j].size()||strs[0][i]!=strs[j][i]){
                    return res;
                }
            }
            res.push_back(strs[0][i]);
        }
        return res;
    }
};
```

## algorithm analysis

其实整个算法的逻辑是不需要判断size==1的，也就是第一行。但是如果不判断的话，就会在循环中一直判断并且调用push\_back到res中，会及其耗时间。加上第一个判断之后，整个代码运行时会快很多的。


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