.
This works because:
The old value is arr[i] % n.
The new value is arr[i] / n (integer division).
Code

[{"language":"C++","code":"class Solution {\npublic:\n\tvoid rearrange(vector<int>& arr) {\n\t\tint n = arr.size();\n\t\tfor (int i = 0; i < n; ++i) {\n\t\t\tarr[i] += (arr[arr[i]] % n) * n;\n\t\t}\n\t\tfor (int i = 0; i < n; ++i) {\n\t\t\tarr[i] /= n;\n\t\t}\n\t}\n};","lang":"cpp","html":"<pre class=\"shiki shiki-themes github-light github-dark\" style=\"background-color:#fff;--shiki-dark-bg:#24292e;color:#24292e;--shiki-dark:#e1e4e8\" tabindex=\"0\"><code><span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">class</span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\"> Solution</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> {</span></span>\n<span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">public:</span></span>\n<span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">\tvoid</span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\"> rearrange</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">(</span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\">vector</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">&#x3C;</span><span style=\"color:#D73A49;--shiki-dark:#F97583\">int</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">></span><span style=\"color:#D73A49;--shiki-dark:#F97583\">&#x26;</span><span style=\"color:#E36209;--shiki-dark:#FFAB70\"> arr</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">) {</span></span>\n<span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">\t\tint</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> n </span><span style=\"color:#D73A49;--shiki-dark:#F97583\">=</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> arr.</span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\">size</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">();</span></span>\n<span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">\t\tfor</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> (</span><span style=\"color:#D73A49;--shiki-dark:#F97583\">int</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> i </span><span style=\"color:#D73A49;--shiki-dark:#F97583\">=</span><span style=\"color:#005CC5;--shiki-dark:#79B8FF\"> 0</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">; i </span><span style=\"color:#D73A49;--shiki-dark:#F97583\">&#x3C;</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> n; </span><span style=\"color:#D73A49;--shiki-dark:#F97583\">++</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">i) {</span></span>\n<span class=\"line\"><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">\t\t\tarr[i] </span><span style=\"color:#D73A49;--shiki-dark:#F97583\">+=</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> (arr[arr[i]] </span><span style=\"color:#D73A49;--shiki-dark:#F97583\">%</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> n) </span><span style=\"color:#
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