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41 lines
2.5 KiB
HTML
41 lines
2.5 KiB
HTML
<p>You are an ant tasked with adding <code>n</code> new rooms numbered <code>0</code> to <code>n-1</code> to your colony. You are given the expansion plan as a <strong>0-indexed</strong> integer array of length <code>n</code>, <code>prevRoom</code>, where <code>prevRoom[i]</code> indicates that you must build room <code>prevRoom[i]</code> before building room <code>i</code>, and these two rooms must be connected <strong>directly</strong>. Room <code>0</code> is already built, so <code>prevRoom[0] = -1</code>. The expansion plan is given such that once all the rooms are built, every room will be reachable from room <code>0</code>.</p>
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<p>You can only build <strong>one room</strong> at a time, and you can travel freely between rooms you have <strong>already built</strong> only if they are <strong>connected</strong>. You can choose to build <strong>any room</strong> as long as its <strong>previous room</strong> is already built.</p>
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<p>Return <em>the <strong>number of different orders</strong> you can build all the rooms in</em>. Since the answer may be large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>
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<p> </p>
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<p><strong class="example">Example 1:</strong></p>
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<img alt="" src="https://assets.leetcode.com/uploads/2021/06/19/d1.JPG" style="width: 200px; height: 212px;" />
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<pre>
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<strong>Input:</strong> prevRoom = [-1,0,1]
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<strong>Output:</strong> 1
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<strong>Explanation:</strong> There is only one way to build the additional rooms: 0 → 1 → 2
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</pre>
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<p><strong class="example">Example 2:</strong></p>
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<strong><img alt="" src="https://assets.leetcode.com/uploads/2021/06/19/d2.JPG" style="width: 200px; height: 239px;" /></strong>
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<pre>
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<strong>Input:</strong> prevRoom = [-1,0,0,1,2]
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<strong>Output:</strong> 6
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<strong>Explanation:
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</strong>The 6 ways are:
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0 → 1 → 3 → 2 → 4
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0 → 2 → 4 → 1 → 3
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0 → 1 → 2 → 3 → 4
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0 → 1 → 2 → 4 → 3
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0 → 2 → 1 → 3 → 4
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0 → 2 → 1 → 4 → 3
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</pre>
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<p> </p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>n == prevRoom.length</code></li>
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<li><code>2 <= n <= 10<sup>5</sup></code></li>
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<li><code>prevRoom[0] == -1</code></li>
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<li><code>0 <= prevRoom[i] < n</code> for all <code>1 <= i < n</code></li>
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<li>Every room is reachable from room <code>0</code> once all the rooms are built.</li>
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</ul> |