<p>Alice has some number of cards and she wants to rearrange the cards into groups so that each group is of size <code>groupSize</code>, and consists of <code>groupSize</code> consecutive cards.</p>

<p>Given an integer array <code>hand</code> where <code>hand[i]</code> is the value written on the <code>i<sup>th</sup></code> card and an integer <code>groupSize</code>, return <code>true</code> if she can rearrange the cards, or <code>false</code> otherwise.</p>

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

<pre>
<strong>Input:</strong> hand = [1,2,3,6,2,3,4,7,8], groupSize = 3
<strong>Output:</strong> true
<strong>Explanation:</strong> Alice&#39;s hand can be rearranged as [1,2,3],[2,3,4],[6,7,8]
</pre>

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

<pre>
<strong>Input:</strong> hand = [1,2,3,4,5], groupSize = 4
<strong>Output:</strong> false
<strong>Explanation:</strong> Alice&#39;s hand can not be rearranged into groups of 4.

</pre>

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

<ul>
	<li><code>1 &lt;= hand.length &lt;= 10<sup>4</sup></code></li>
	<li><code>0 &lt;= hand[i] &lt;= 10<sup>9</sup></code></li>
	<li><code>1 &lt;= groupSize &lt;= hand.length</code></li>
</ul>

<p>&nbsp;</p>
<p><strong>Note:</strong> This question is the same as 1296: <a href="https://leetcode.com/problems/divide-array-in-sets-of-k-consecutive-numbers/" target="_blank">https://leetcode.com/problems/divide-array-in-sets-of-k-consecutive-numbers/</a></p>

class Solution {
public:
    bool isNStraightHand(vector<int>& hand, int groupSize) {
        if (hand.size() % groupSize != 0) { return false; }
        if (groupSize == 1) { return true; }

        ranges::sort(hand);
        auto offset = hand.begin();
        auto end = hand.end();
        constexpr int MARKER = INT_MAX;

        while (offset != end) {
            auto it = offset;
            int num = *it;
            int count = 1;
            while (++it != end && *it == num) {
                ++count;
            }

            offset = it;
            for (int i = 1; i < groupSize; ++i) {
                int remaining = count;
                int expected = num + i;
                while (remaining > 0) {
                    if (it == end || *it != expected) {
                        return false;
                    }
                    *it = MARKER;
                    --remaining;
                    ++it;
                }
                while (it != end && *it == expected) {
                    ++it;
                }
            }

            --it;
            auto insert = it;
            auto take = it;
            while (take >= offset) {
                if (*take != MARKER) {
                    *insert = *take;
                    --insert;
                }
                --take;
            }
            offset = insert;
            ++offset; // `insert` is where we *would* insert the next element
        }
        return true;
    }
};

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