<p>Write a function to find the longest common prefix string amongst an array of strings.</p>

<p>If there is no common prefix, return an empty string <code>&quot;&quot;</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> strs = [&quot;flower&quot;,&quot;flow&quot;,&quot;flight&quot;]
<strong>Output:</strong> &quot;fl&quot;
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> strs = [&quot;dog&quot;,&quot;racecar&quot;,&quot;car&quot;]
<strong>Output:</strong> &quot;&quot;
<strong>Explanation:</strong> There is no common prefix among the input strings.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= strs.length &lt;= 200</code></li>
	<li><code>0 &lt;= strs[i].length &lt;= 200</code></li>
	<li><code>strs[i]</code> consists of only lowercase English letters.</li>
</ul>

class Solution {
public:
    string common(string a, string b) {
        string ans = "";
        int i = 0;
        while(i < a.size() && i< b.size()) {
            if (a[i] != b[i])break;
            ans += a[i];
            i++;
        }
        return ans;
    }

    string calculate(int start, int end, vector<string>& strs) {
        // cout <<"start="<<start<<" end="<<end<<endl;
        int diff = (end-start)/2;
        if (end - start == 1) return strs[start];
        string ans1 = calculate(start,start+diff,strs);
        string ans2 = calculate(start+diff,end,strs);
        string ans = common(ans1,ans2);
        return ans;
    }

    string longestCommonPrefix(vector<string>& strs) {
        string ans = calculate(0, strs.size(), strs);
        return ans;
    }
};
15800