The task is as follows:
For a given value ‘x’ we have to quickly find the smallest index ‘i’ such that the sum of the first ‘i’ elements of the array a is greater or equal to x (assuming that the array a only contains non-negative values).
This task can be solved using binary search, computing the sum of the prefixes with the Segment Tree. However, this will lead to a O((logn)^2) solution.
Instead we can use the same idea as in the previous section, and find the position by descending the tree: by moving each time to the left or the right, depending on the sum of the left child. Thus finding the answer in O(logn) time.
this is taken from : https://cp-algorithms.com/data_structures/segment_tree.html
but I can’t understand the second paragraph the one which says about binary search. Can someone explain to me how to get the complexity of (log(n))^2