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src/euler45/Cargo.toml

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+[package]
+name = "euler45"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]

src/euler45/src/main.rs

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+use std::collections::HashSet;
+
+fn main() {
+    let triangle_numbers: Vec<_> = (286..100_000).map(triangle).collect();
+    let pentagonal_numbers: HashSet<_> = (2..100_000).map(pentagonal).collect();
+    let hexagonal_numbers: HashSet<_> = (2..100_000).map(hexagonal).collect();
+
+    for t in triangle_numbers {
+        if pentagonal_numbers.contains(&t) && hexagonal_numbers.contains(&t) {
+            println!("{t}");
+            break;
+        }
+    }
+}
+
+fn triangle(n: usize) -> usize {
+    n * (n + 1) / 2
+}
+
+fn pentagonal(n: usize) -> usize {
+    n * (3 * n - 1) / 2
+}
+
+fn hexagonal(n: usize) -> usize {
+    n * (2 * n - 1)
+}