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leetcode-cn/problem (English)/数组归约运算(English) [array-reduce-transformation].html

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+<p>Given an integer array&nbsp;<code>nums</code>, a reducer function&nbsp;<code>fn</code>, and an intial value&nbsp;<code>init</code>, return a&nbsp;<strong>reduced</strong>&nbsp;array.</p>
+
+<p>A&nbsp;<strong>reduced</strong>&nbsp;array is created by applying the following operation:&nbsp;<code>val = fn(init, nums[0])</code>, <code>val&nbsp;= fn(val, nums[1])</code>,&nbsp;<code>val&nbsp;= fn(val, arr[2])</code>,&nbsp;<code>...</code>&nbsp;until every element in the array has been processed. The final value of&nbsp;<code>val</code>&nbsp;is returned.</p>
+
+<p>If the length of the array is 0, it should return&nbsp;<code>init</code>.</p>
+
+<p>Please solve it without using the built-in <code>Array.reduce</code> method.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> 
+nums = [1,2,3,4]
+fn = function sum(accum, curr) { return accum + curr; }
+init = 0
+<strong>Output:</strong> 10
+<strong>Explanation:</strong>
+initially, the value is init=0.
+(0) + nums[0] = 1
+(1) + nums[1] = 3
+(3) + nums[2] = 6
+(6) + nums[3] = 10
+The final answer is 10.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> 
+nums = [1,2,3,4]
+fn = function sum(accum, curr) { return accum + curr * curr; }
+init = 100
+<strong>Output:</strong> 130
+<strong>Explanation:</strong>
+initially, the value is init=100.
+(100) + nums[0]^2 = 101
+(101) + nums[1]^2 = 105
+(105) + nums[2]^2 = 114
+(114) + nums[3]^2 = 130
+The final answer is 130.
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> 
+nums = []
+fn = function sum(accum, curr) { return 0; }
+init = 25
+<strong>Output:</strong> 25
+<strong>Explanation:</strong> For empty arrays, the answer is always init.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>0 &lt;= nums.length &lt;= 1000</code></li>
+	<li><code>0 &lt;= nums[i] &lt;= 1000</code></li>
+	<li><code>0 &lt;= init &lt;= 1000</code></li>
+</ul>