Add README and solution for day4

day4/rust/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "day4"
+version = "0.1.0"
+authors = ["Tobias Eidelpes <tobias@eidelpes.info>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+itertools = "0.8"

day4/rust/src/main.rs

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+use itertools::Itertools;
+
+fn main() {
+    let passwords = (152_086_usize..670_282_usize)
+        .map(|x| x.to_string().chars().collect::<Vec<char>>())
+        .filter(|x| x.iter().zip(x.iter().skip(1)).all(|(x,y)| x <= y))
+        .filter(|x| {
+            for (_,g) in &x.iter().group_by(|&y| y) {
+                if g.count() == 2 {
+                    return true;
+                }
+            }
+            false
+        })
+    .count();
+
+    println!("Solution: {:?}", passwords);
+}