Given a sorted array and a target, find the index of the target or return −1 if not found.
Example: Find 7 in [1,3,5,7,9,11,13].
1. low=0, high=6, mid=3. arr[3]=7. Found!
Example: Find 6 in the same array.
1. mid=3, arr[3]=7>6. Search [0,2].
2. mid=1, arr[1]=3<6. Search [2,2].
3. mid=2, arr[2]=5<6. Search [3,2].
4. low>high. Not found. Return −1.
Each comparison eliminates half the remaining elements.