## Basic Scenario

Find the index of a **target matching** element in a **sorted** (either ascending or non-increasing) array.

## Classic examples

Given an array of integers nums which is sorted in ascending order, and an integer target, write a function to search target in nums. If target exists, then return its index. Otherwise, return -1.

You must write an algorithm with O(log n) runtime complexity.

Example 1:

Input: nums = [-1,0,3,5,9,12], target = 9

Output: 4

Explanation: 9 exists in nums and its index is 4

Example 2:

Input: nums = [-1,0,3,5,9,12], target = 2

Output: -1

Explanation: 2 does not exist in nums so return -1

Constraints:

1 <= nums.length <= 104

-104 < nums[i], target < 104

All the integers in nums are unique.

nums is sorted in ascending order.

1 | def binarySearch(nums,target): |

**lc 35 Insert target**

search ends at l = r+1, so finally return l if not found (or r+1).

1 | def searchInsert(nums,target): |

## Summary:

- Both arrays are in an ascending order (no duplicates).
- There are usually two representations for intervals:
`[left, right]`

and`[left, right)`

. I prefer the former, so all the binary search code in blogs is based on the`[left, right]`

format.

Next blog: How to find left and right borders of target in a non-inscreading array. How to search target in unsual array.