First Day Where You Have Been in All the Rooms
<p>There are <code>n</code> rooms you need to visit, labeled from <code>0</code> to <code>n - 1</code>. Each day is labeled, starting from <code>0</code>. You will go in and visit one room a day.</p>

<p>Initially on day <code>0</code>, you visit room <code>0</code>. The <strong>order</strong> you visit the rooms for the coming days is determined by the following <strong>rules</strong> and a given <strong>0-indexed</strong> array <code>nextVisit</code> of length <code>n</code>:</p>

<ul>
	<li>Assuming that on a day, you visit room <code>i</code>,</li>
	<li>if you have been in room <code>i</code> an <strong>odd</strong> number of times (<strong>including</strong> the current visit), on the <strong>next</strong> day you will visit a room with a <strong>lower or equal room number</strong> specified by <code>nextVisit[i]</code> where <code>0 &lt;= nextVisit[i] &lt;= i</code>;</li>
	<li>if you have been in room <code>i</code> an <strong>even</strong> number of times (<strong>including</strong> the current visit), on the <strong>next</strong> day you will visit room <code>(i + 1) mod n</code>.</li>
</ul>

<p>Return <em>the label of the <strong>first</strong> day where you have been in <strong>all</strong> the rooms</em>. It can be shown that such a day exists. Since the answer may be very large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> nextVisit = [0,0]
<strong>Output:</strong> 2
<strong>Explanation:</strong>
- On day 0, you visit room 0. The total times you have been in room 0 is 1, which is odd.
&nbsp; On the next day you will visit room nextVisit[0] = 0
- On day 1, you visit room 0, The total times you have been in room 0 is 2, which is even.
&nbsp; On the next day you will visit room (0 + 1) mod 2 = 1
- On day 2, you visit room 1. This is the first day where you have been in all the rooms.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> nextVisit = [0,0,2]
<strong>Output:</strong> 6
<strong>Explanation:</strong>
Your room visiting order for each day is: [0,0,1,0,0,1,2,...].
Day 6 is the first day where you have been in all the rooms.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> nextVisit = [0,1,2,0]
<strong>Output:</strong> 6
<strong>Explanation:</strong>
Your room visiting order for each day is: [0,0,1,1,2,2,3,...].
Day 6 is the first day where you have been in all the rooms.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>n == nextVisit.length</code></li>
	<li><code>2 &lt;= n &lt;= 10<sup>5</sup></code></li>
	<li><code>0 &lt;= nextVisit[i] &lt;= i</code></li>
</ul>
Array; Dynamic Programming
class Solution {
public:
    int firstDayBeenInAllRooms(vector<int>& nextVisit) {
        int n = nextVisit.size();
        vector<long long> f(n);
        const int mod = 1e9 + 7;
        for (int i = 1; i < n; ++i) {
            f[i] = (f[i - 1] + 1 + f[i - 1] - f[nextVisit[i - 1]] + 1 + mod) % mod;
        }
        return f[n - 1];
    }
};