rs: add «2485. Find the Pivot Integer»

Signed-off-by: Matej Focko <mfocko@redhat.com>

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);
+    }
+}