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53 lines
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53 lines
2.5 KiB
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<p><strong>特殊序列</strong> 是由 <strong>正整数</strong> 个 <code>0</code> ，紧接着 <strong>正整数</strong> 个 <code>1</code> ，最后 <strong>正整数</strong> 个 <code>2</code> 组成的序列。</p>
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<ul>
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<li>比方说，<code>[0,1,2]</code> 和 <code>[0,0,1,1,1,2]</code> 是特殊序列。</li>
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<li>相反，<code>[2,1,0]</code> ，<code>[1]</code> 和 <code>[0,1,2,0]</code> 就不是特殊序列。</li>
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</ul>
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<p>给你一个数组 <code>nums</code> （<strong>仅</strong> 包含整数 <code>0</code>，<code>1</code> 和 <code>2</code>），请你返回 <b>不同特殊子序列的数目</b> 。由于答案可能很大，请你将它对 <code>10<sup>9</sup> + 7</code> <strong>取余</strong> 后返回。</p>
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<p>一个数组的 <strong>子序列</strong> 是从原数组中删除零个或者若干个元素后，剩下元素不改变顺序得到的序列。如果两个子序列的 <strong>下标集合</strong> 不同，那么这两个子序列是 <strong>不同的</strong> 。</p>
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<p> </p>
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<p><strong>示例 1：</strong></p>
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<pre>
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<b>输入：</b>nums = [0,1,2,2]
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<b>输出：</b>3
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<b>解释：</b>特殊子序列为 [<strong>0</strong>,<strong>1</strong>,<strong>2</strong>,2]，[<strong>0</strong>,<strong>1</strong>,2,<strong>2</strong>] 和 [<strong>0</strong>,<strong>1</strong>,<strong>2</strong>,<strong>2</strong>] 。
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</pre>
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<p><strong>示例 2：</strong></p>
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<pre>
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<b>输入：</b>nums = [2,2,0,0]
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<b>输出：</b>0
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<b>解释：</b>数组 [2,2,0,0] 中没有特殊子序列。
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</pre>
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<p><strong>示例 3：</strong></p>
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<pre>
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<b>输入：</b>nums = [0,1,2,0,1,2]
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<b>输出：</b>7
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<b>解释：</b>特殊子序列包括：
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- [<strong>0</strong>,<strong>1</strong>,<strong>2</strong>,0,1,2]
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- [<strong>0</strong>,<strong>1</strong>,2,0,1,<strong>2</strong>]
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- [<strong>0</strong>,<strong>1</strong>,<strong>2</strong>,0,1,<strong>2</strong>]
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- [<strong>0</strong>,<strong>1</strong>,2,0,<strong>1</strong>,<strong>2</strong>]
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- [<strong>0</strong>,1,2,<strong>0</strong>,<strong>1</strong>,<strong>2</strong>]
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- [<strong>0</strong>,1,2,0,<strong>1</strong>,<strong>2</strong>]
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- [0,1,2,<strong>0</strong>,<strong>1</strong>,<strong>2</strong>]
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</pre>
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<p> </p>
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<p><strong>提示：</strong></p>
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<ul>
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<li><code>1 <= nums.length <= 10<sup>5</sup></code></li>
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<li><code>0 <= nums[i] <= 2</code></li>
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</ul>
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