Magic Index

A magic index in an array A[0..n-1] is defined such that A[i] == i.

Given a sorted array of distinct integers, write a method to find a magic index if one exists.

Examples:

[-10, -5, 0, 3, 7]  → 3   (A[3] == 3)
[0, 4, 5, 6, 7]     → 0   (A[0] == 0)
[-10, -9, -2, -1]   → None

Follow-up: What if values are not distinct?