# 14. Longest Common Prefix

## 1.問題

![](https://901207480-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LGKoChvN9am4__HCIRK%2F-LLjWXvDGBCXZdyo8Tty%2F-LL_2vwbzRf-cSNo9Oqr%2F%E8%9E%A2%E5%B9%95%E5%BF%AB%E7%85%A7%202018-09-04%20%E4%B8%8B%E5%8D%8810.49.37.png?alt=media\&token=5340fb52-a597-46c5-90da-cf94e4422710)

## 2.想法

* 提問
* function header, parameter
* test input
* 說明想法
  * 用vector\<int> 儲存每個string各字母出現的次數, 當相同字母的出現次數與字串數量相同時, increment + 1
* 測試計算複雜度: O(n)

## **3.程式碼**

```
class Solution {
public:
    string longestCommonPrefix(vector<string>& strs) {
        int size = strs.size();
        if (size == 0) {
            return "";
        }
        vector<char> v(128, 0);
        string s = "";
        int maxNum = INT_MAX;
        for (int i = 0; i < size; i++) {
            maxNum = min(maxNum, int(strs[i].size()));
        }

        for (int j = 0; j < maxNum; j++) {
            for (int i = 0; i < size; i++) {
                v[int(strs[i][j] - '0')]++;
            }
            if (v[int(strs[0][j] - '0')] == size) {
                s.push_back(strs[0][j]);
                v[int(strs[0][j] - '0')] = 0;
            } else {
                return s;
            }
        }
        return s;
    }
};
```