feat: add solution 3021. Alice and Bob Playing Flower Game

Author: tzuyi0817

solutions/3021. Alice and Bob Playing Flower Game/README.md

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+# [3021. Alice and Bob Playing Flower Game](https://leetcode.com/problems/alice-and-bob-playing-flower-game)
+
+## Description
+
+<div class="elfjS" data-track-load="description_content"><p>Alice and Bob are playing a turn-based game on a field, with two lanes of flowers between them. There are <code>x</code> flowers in the first lane between Alice and Bob, and <code>y</code> flowers in the second lane between them.</p>
+
+<p><img alt="" src="https://assets.leetcode.com/uploads/2025/08/27/3021.png" style="width: 300px; height: 150px;"></p>
+
+<p>The game proceeds as follows:</p>
+
+<ol>
+	<li>Alice takes the first turn.</li>
+	<li>In each turn, a player must choose either one of the lane&nbsp;and pick one flower from that side.</li>
+	<li>At the end of the turn, if there are no flowers left at all, the <strong>current</strong> player captures their opponent and wins the game.</li>
+</ol>
+
+<p>Given two integers, <code>n</code> and <code>m</code>, the task is to compute the number of possible pairs <code>(x, y)</code> that satisfy the conditions:</p>
+
+<ul>
+	<li>Alice must win the game according to the described rules.</li>
+	<li>The number of flowers <code>x</code> in the first lane must be in the range <code>[1,n]</code>.</li>
+	<li>The number of flowers <code>y</code> in the second lane must be in the range <code>[1,m]</code>.</li>
+</ul>
+
+<p>Return <em>the number of possible pairs</em> <code>(x, y)</code> <em>that satisfy the conditions mentioned in the statement</em>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre><strong>Input:</strong> n = 3, m = 2
+<strong>Output:</strong> 3
+<strong>Explanation:</strong> The following pairs satisfy conditions described in the statement: (1,2), (3,2), (2,1).
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre><strong>Input:</strong> n = 1, m = 1
+<strong>Output:</strong> 0
+<strong>Explanation:</strong> No pairs satisfy the conditions described in the statement.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= n, m &lt;= 10<sup>5</sup></code></li>
+</ul>
+</div>
+
+<p>&nbsp;</p>
+
+## Solutions
+
+**Solution: `Math`**
+
+- Time complexity: <em>O(1)</em>
+- Space complexity: <em>O(1)</em>
+
+<p>&nbsp;</p>
+
+### **JavaScript**
+
+```js
+/**
+ * @param {number} n
+ * @param {number} m
+ * @return {number}
+ */
+const flowerGame = function (n, m) {
+  const countEvenN = Math.floor(n / 2);
+  const countOddN = n - countEvenN;
+  const countEvenM = Math.floor(m / 2);
+  const countOddM = m - countEvenM;
+
+  return countOddN * countEvenM + countEvenN * countOddM;
+};
+```

solutions/3021. Alice and Bob Playing Flower Game/solution.js

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+/**
+ * @param {number} n
+ * @param {number} m
+ * @return {number}
+ */
+const flowerGame = function (n, m) {
+  const countEvenN = Math.floor(n / 2);
+  const countOddN = n - countEvenN;
+  const countEvenM = Math.floor(m / 2);
+  const countOddM = m - countEvenM;
+
+  return countOddN * countEvenM + countEvenN * countOddM;
+};