From 963340d14a960f86e22bbbf0ee6f40bde843da1e Mon Sep 17 00:00:00 2001
From: Philippe Zwietering <philippe.zwietering@gmail.com>
Date: Fri, 28 Apr 2023 17:06:07 +0200
Subject: [PATCH] abc050 c finished

---
 atcoder/beginner_contests/abc050/src/bin/c.rs | 60 ++++++++++++++++++-
 1 file changed, 58 insertions(+), 2 deletions(-)

diff --git a/atcoder/beginner_contests/abc050/src/bin/c.rs b/atcoder/beginner_contests/abc050/src/bin/c.rs
index ee80563..fdbe32b 100644
--- a/atcoder/beginner_contests/abc050/src/bin/c.rs
+++ b/atcoder/beginner_contests/abc050/src/bin/c.rs
@@ -1,3 +1,59 @@
-fn main() {
-    unimplemented!();
+use std::collections::HashMap;
+
+use proconio::input;
+
+const MAX_RESULT: u64 = 1000000007;
+
+fn calc_orders(ans: &[u64]) -> u64 {
+    let mut visited_positions: HashMap<u64, usize> = HashMap::new();
+    let mut result = 1;
+    let l = ans.len();
+
+    // For all reports, it needs to be at least 0 and at most n - 1.
+    if l % 2 == 0 {
+        for &a in ans.into_iter() {
+            // In this case, all reports have to be odd and occur twice.
+            // We don't have to check if a < 0 because a is unsigned and rust will panic during the
+            // parsing of the input anyways.
+            if a % 2 != 1 || a >= l as u64 {
+                return 0;
+            }
+
+            let number_of_reports = *visited_positions.get(&a).unwrap_or(&0);
+            if number_of_reports > 1 {
+                return 0;
+            } else if number_of_reports == 0 {
+                result = (result * 2) % MAX_RESULT;
+            }
+            visited_positions.insert(a, number_of_reports + 1);
+        }
+    } else if l % 2 == 1 {
+        // In this case, all reports have to be even, occur twice, except 0, which needs to occur
+        // exactly once.
+        for &a in ans.into_iter() {
+            if a % 2 != 0 || a >= l as u64 {
+                return 0;
+            }
+
+            let number_of_reports = *visited_positions.get(&a).unwrap_or(&0);
+            if number_of_reports > 1 || (number_of_reports > 0 && a == 0) {
+                return 0;
+            } else if a != 0 && number_of_reports == 0 {
+                result = (result * 2) % MAX_RESULT;
+            }
+
+            visited_positions.insert(a, number_of_reports + 1);
+        }
+    }
+
+    return result;
+}
+
+fn main() {
+    input! {
+        n: usize,
+        ans: [u64; n],
+    };
+
+    println!("{}", calc_orders(&ans));
 }