The best answer by @gatecse talks about an array. Don’t be confused there is no array in question the reference is used only to make the Visualization of the concept easier.

Eg. **finding cube root of 1000**, there is no array. So lets try to find out the cube root manually starting from 1, 2, 3, 4……. this will take O(n) time. but since we know the beginning(1) and end(1000) and also logically they are sorted we can make use of binary search concept to get to our answer.