past202206-open e

bouzuya · Jan 17, 2023 · 97f612c · 97f612c
1 parent 55e36f8
commit 97f612c
Showing 1 changed file with 35 additions and 3 deletions.
diff --git a/past202206-open/src/bin/e.rs b/past202206-open/src/bin/e.rs
@@ -1,10 +1,42 @@
-use proconio::{input, marker::Usize1};
+use proconio::input;
 
 fn main() {
     input! {
         n: usize,
-        a: [Usize1; n],
     };
-    let ans = n - a.len();
+
+    if n == 1 {
+        println!("1");
+        return;
+    }
+
+    // 1         =     1 =      1^2
+    // 1+3       =     4 =      2^2
+    // 1+3+5     =     9 =      3^2
+    // 1+3+5+7   =    16 =      4^2
+    // 1+3+5+7+9 =    25 =      5^2
+    // ...       = 10^18 = (10^9)^2
+    let mut ng = 1_usize;
+    let mut ok = 1_000_000_000_usize;
+    while ok - ng > 1 {
+        let mid = ng + (ok - ng) / 2;
+        if n <= mid.pow(2) {
+            ok = mid;
+        } else {
+            ng = mid;
+        }
+    }
+
+    let left = ok - 1;
+    let right = ok;
+    let len = right.pow(2) - left.pow(2);
+    let index = n - left.pow(2) - 1;
+    // 1 , 2 1 2 , 3 2 1 2 3 , 4 3 2 1 2 3 4
+    // 1       4           9              16
+    let ans = if index <= len / 2 {
+        right - index
+    } else {
+        right - len / 2 + index - (len / 2)
+    };
     println!("{}", ans);
 }