feat(solutions): add sorting-algorithms/rust/merge-sort

challenges/sorting-algorithms/solutions/rust/merge-sort/README.md

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+# sorting-algorithms/rust/merge-sort
+
+Created by [@Divlo](https://github.com/Divlo) on 8 March 2022.

challenges/sorting-algorithms/solutions/rust/merge-sort/solution.rs

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+use std::io::{self, BufRead};
+
+fn main() {
+  let mut numbers: Vec<i64> = Vec::new();
+  let stdin = io::stdin();
+  for line in stdin.lock().lines().skip(1) {
+    let line = line.unwrap();
+    let number: i64 = line.trim().parse().unwrap();
+    numbers.push(number);
+  }
+  merge_sort(&mut numbers);
+  for number in numbers {
+    println!("{}", number);
+  }
+}
+
+pub fn merge_sort(array: &mut Vec<i64>) {
+  if array.len() > 1 {
+    let mid = array.len() / 2;
+    let mut left = array[..mid].to_vec();
+    let mut right = array[mid..].to_vec();
+    merge_sort(&mut left);
+    merge_sort(&mut right);
+    merge(left, right, array);
+  }
+}
+
+pub fn merge(left: Vec<i64>, right: Vec<i64>, array: &mut Vec<i64>) {
+  let mut left_index = 0;
+  let mut right_index = 0;
+  let mut array_index = 0;
+  while left_index < left.len() && right_index < right.len() {
+    if left[left_index] <= right[right_index] {
+      array[array_index] = left[left_index];
+      left_index += 1;
+    } else {
+      array[array_index] = right[right_index];
+      right_index += 1;
+    }
+    array_index += 1;
+  }
+  while left_index < left.len() {
+    array[array_index] = left[left_index];
+    left_index += 1;
+    array_index += 1;
+  }
+  while right_index < right.len() {
+    array[array_index] = right[right_index];
+    right_index += 1;
+    array_index += 1;
+  }
+}