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leetcode-cn/problem (English)/分组(English) [group-by].html

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+<p>Write code that enhances all arrays such that you can call the&nbsp;<code>array.groupBy(fn)</code>&nbsp;method on any array and it will return a <strong>grouped</strong>&nbsp;version of the array.</p>
+
+<p>A&nbsp;<strong>grouped</strong>&nbsp;array is an object where each&nbsp;key&nbsp;is&nbsp;the output of&nbsp;<code>fn(arr[i])</code>&nbsp;and each&nbsp;value is an array containing all items in the original array with that key.</p>
+
+<p>The provided callback&nbsp;<code>fn</code>&nbsp;will accept an item in the array and return a string key.</p>
+
+<p>The order of each value list should be the order the items&nbsp;appear in the array. Any order of keys is acceptable.</p>
+
+<p>Please solve it without lodash&#39;s&nbsp;<code>_.groupBy</code> function.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> 
+array = [
+&nbsp; {&quot;id&quot;:&quot;1&quot;},
+&nbsp; {&quot;id&quot;:&quot;1&quot;},
+&nbsp; {&quot;id&quot;:&quot;2&quot;}
+], 
+fn = function (item) { 
+&nbsp; return item.id; 
+}
+<strong>Output:</strong> 
+{ 
+&nbsp; &quot;1&quot;: [{&quot;id&quot;: &quot;1&quot;}, {&quot;id&quot;: &quot;1&quot;}], &nbsp; 
+&nbsp; &quot;2&quot;: [{&quot;id&quot;: &quot;2&quot;}] 
+}
+<strong>Explanation:</strong>
+Output is from array.groupBy(fn).
+The selector function gets the &quot;id&quot; out of each item in the array.
+There are two objects with an &quot;id&quot; of 1. Both of those objects are put in the first array.
+There is one object with an &quot;id&quot; of 2. That object is put in the second array.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> 
+array = [
+&nbsp; [1, 2, 3],
+&nbsp; [1, 3, 5],
+&nbsp; [1, 5, 9]
+]
+fn = function (list) { 
+&nbsp; return String(list[0]); 
+}
+<strong>Output:</strong> 
+{ 
+&nbsp; &quot;1&quot;: [[1, 2, 3], [1, 3, 5], [1, 5, 9]] 
+}
+<strong>Explanation:</strong>
+The array can be of any type. In this case, the selector function defines the key as being the first element in the array. 
+All the arrays have 1 as their first element so they are grouped together.
+{
+  &quot;1&quot;: [[1, 2, 3], [1, 3, 5], [1, 5, 9]]
+}
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> 
+array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+fn = function (n) { 
+&nbsp; return String(n &gt; 5);
+}
+<strong>Output:</strong>
+{
+&nbsp; &quot;true&quot;: [6, 7, 8, 9, 10],
+&nbsp; &quot;false&quot;: [1, 2, 3, 4, 5]
+}
+<strong>Explanation:</strong>
+The selector function splits the array by whether each number is greater than 5.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>0 &lt;= array.length &lt;= 10<sup>5</sup></code></li>
+	<li><code>fn returns a string</code></li>
+</ul>