missing number solution added

2023_09_18/missing_number/Cargo.toml

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+[package]
+name = "missing_number"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2023_09_18/missing_number/README.md

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+# Comments on the solution
+
+We are using a simple mathematical approach that it's pretty much self-explanatory. We can express the sum of all numbers from `0` to `n` as `n * (n + 1) / 2`. We can then subtract the sum of the array from the sum of all numbers from `0` to `n` to get the missing number.
+
+### Complexity
+
+- **Time Complexity:** `O(n)` - We are iterating over the array once.
+- **Space Complexity:** `O(1)` - We are not using any extra space.
+
+---
+
+### Solution in Rust
+
+```rust
+fn missing_number(nums: Vec<i32>) -> i32 {
+    (1..=nums.len() as i32).sum::<i32>() - nums.iter().sum::<i32>()
+}
+```
+
+where we use the `..=` range operator to create a range from `1` to the length of the input vector, and then sum the values in that range using the `sum` method
+
+### Leetcode result
+
+![](https://i.imgur.com/qW2tNVm.png)

2023_09_18/missing_number/src/main.rs

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+// Given an array nums containing n distinct numbers in the range [0, n], return the only number in the range that is missing from the array.
+
+fn missing_number(nums: Vec<i32>) -> i32 {
+    (1..=nums.len() as i32).sum::<i32>() - nums.iter().sum::<i32>()
+}
+
+fn main() {
+    let nums = vec![3, 0, 1];
+    println!("{}", missing_number(nums));
+}