From 8f865fa34c11b19168261ddb66ed5bf861a0d2f5 Mon Sep 17 00:00:00 2001
From: Matej Focko <mfocko@redhat.com>
Date: Wed, 13 Mar 2024 22:25:22 +0100
Subject: [PATCH] =?UTF-8?q?rs:=20add=20=C2=AB2485.=20Find=20the=20Pivot=20?=
 =?UTF-8?q?Integer=C2=BB?=
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Signed-off-by: Matej Focko <mfocko@redhat.com>
---
 rs/find-the-pivot-integer.rs | 43 ++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)
 create mode 100644 rs/find-the-pivot-integer.rs

diff --git a/rs/find-the-pivot-integer.rs b/rs/find-the-pivot-integer.rs
new file mode 100644
index 0000000..c671e87
--- /dev/null
+++ b/rs/find-the-pivot-integer.rs
@@ -0,0 +1,43 @@
+struct Solution {}
+impl Solution {
+    pub fn pivot_integer(n: i32) -> i32 {
+        // S_i = S_n - S_{i - 1}
+        // i * (1 + i) / 2 = n * (1 + n) / 2 - i * (i - 1) / 2
+        // i * (1 + i) = n * (1 + n) - i * (i - 1)
+        // 2i² = n(1 + n)
+        // i = sqrt(n * (1 + n) / 2)
+
+        let expected_ii = n * (1 + n) / 2;
+
+        let mut i = 1;
+        while i * i < expected_ii {
+            i += 1;
+        }
+
+        if i * i == expected_ii {
+            return i;
+        }
+
+        -1
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn example_1() {
+        assert_eq!(Solution::pivot_integer(8), 6);
+    }
+
+    #[test]
+    fn example_2() {
+        assert_eq!(Solution::pivot_integer(1), 1);
+    }
+
+    #[test]
+    fn example_3() {
+        assert_eq!(Solution::pivot_integer(4), -1);
+    }
+}