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Binary Search (BS) is, arguably, the most popular question in coding interviews. Even when you see that a problem can be solved with BS, it can be tricky to handle all of the indexes and base cases in the solution. What if there was an approach that could work for all BS problems? 
This course was designed from scratch with this one goal in mind. We'll explore one technique that can handle any BS problem. The practice problems in this course were carefully chosen to cover the most frequently asked BS questions in coding interviews. At the end of this course, you will feel confident to handle BS questions during phone or on-site interviews, as well as with all of the follow-ups about its time and space complexity.

Binary Search for Coding Interviews

## Problem statement

Suppose we are given a **sorted array** of distinct integers, `nums`, which are sorted in ascending order. We will return the **smallest index** `i`, which satisfies `nums[i] == i`. If no such `i` exists, we will return `-1`.

**Constraints:**

- $1 \leq nums.length \leq 10^3$
- $-10^3 \leq nums[i] \leq 10^3$

**Example 1: General case**
```
Input: [-10,-5,0,3,7]    
Output: 3    
```
**Explanation:** For the given array, `nums[0] = -10`, `nums[1] = -5`, `nums[2] = 0`, `nums[3] = 3`, the output is `3`.

**Example 2: The answer is at the beginning of the array**
```
Input: [0,2,5,8,17]    
Output: 0    
```
**Explanation:** `nums[0] = 0`. Thus the output is `0`.

**Example 3: The answer is missing**
```
Input: [-10,-5,3,4,7,9]    
Output: -1    
```
**Explanation:** There is no such `i` that `nums[i] = i`. Thus the output is `-1`.

# Problem statement

Suppose we are given a **sorted array** of distinct integers, `nums`, which are sorted in ascending order. We will return the **smallest index** `i`, which satisfies `nums[i] == i`. If no such `i` exists, we will return `-1`.

**Constraints:**

- $1 \leq nums.length \leq 10^3$
- $-10^3 \leq nums[i] \leq 10^3$

**Example 1: General case**
```
Input: [-10,-5,0,3,7]    
Output: 3    
```
**Explanation:** For the given array, `nums[0] = -10`, `nums[1] = -5`, `nums[2] = 0`, `nums[3] = 3`, the output is `3`.

**Example 2: The answer is at the beginning of the array**
```
Input: [0,2,5,8,17]    
Output: 0    
```
**Explanation:** `nums[0] = 0`. Thus the output is `0`.

**Example 3: The answer is missing**
```
Input: [-10,-5,3,4,7,9]    
Output: -1    
```
**Explanation:** There is no such `i` that `nums[i] = i`. Thus the output is `-1`.

Let's check another problem with a sorted input array.

Difficulty Level: Easy

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First Element Equals its Index

Problem statement