Given an array of $n$ integers, print the summation of elements which are at prime indices

$O(n^2)$ will land you in TLE $,$ Your Algorithm should be lesser than or equal to $*\sqrt[\phantom{2}]{n})$

time complexity

Given a binary tree, find the sum of alternate level nodes

Algorithm should be less than or equal to $O(n)$ time complexity, where $n$ is the number of

nodes..GIVE COMPLETE ANSWER FOR THE THREE QNS IN THE IMAGE VERY URGENT PLS