init

problems/p21/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "p21"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]

problems/p21/src/main.rs

@@ -0,0 +1,33 @@
+fn d(n: u64) -> u64 {
+    let mut sum = 1u64;
+    let sqrt_n = (n as f64).sqrt() as u64;
+
+    for i in 2..=sqrt_n {
+        if n % i == 0 {
+            sum += i;
+            if i != n / i {
+                // 避免完全平方数重复计算
+                sum += n / i;
+            }
+        }
+    }
+    sum
+}
+
+fn main() {
+    let mut amicables: Vec<u64> = Vec::new();
+    for i in 2..=10000 {
+        if amicables.contains(&i) {
+            continue;
+        }
+        let pair = d(i);
+        if d(pair) == i && pair != i {
+            // 排除完美数
+            amicables.push(i);
+            amicables.push(pair);
+        }
+    }
+    let sum: u64 = amicables.iter().sum();
+    println!("{sum}");
+}
+