diff --git a/src/general/mex.rs b/src/general/mex.rs
new file mode 100644
index 00000000000..aa79ea1fe60
--- /dev/null
+++ b/src/general/mex.rs
@@ -0,0 +1,79 @@
+use std::collections::BTreeSet;
+
+// Find minimum excluded number from a set of given numbers using a set
+/// Finds the MEX of the values provided in `arr`
+/// Uses [`BTreeSet`](std::collections::BTreeSet)
+/// O(nlog(n)) implementation
+pub fn mex_using_set(arr: &[i64]) -> i64 {
+    let mut s: BTreeSet<i64> = BTreeSet::new();
+    for i in 0..arr.len() + 1 {
+        s.insert(i as i64);
+    }
+    for x in arr {
+        s.remove(x);
+    }
+    // TODO: change the next 10 lines to *s.first().unwrap() when merged into stable
+    // set should never have 0 elements
+    if let Some(x) = s.into_iter().next() {
+        x
+    } else {
+        panic!("Some unknown error in mex_using_set")
+    }
+}
+/// Finds the MEX of the values provided in `arr`
+/// Uses sorting
+/// O(nlog(n)) implementation
+pub fn mex_using_sort(arr: &[i64]) -> i64 {
+    let mut arr = arr.to_vec();
+    arr.sort();
+    let mut mex = 0;
+    for x in arr {
+        if x == mex {
+            mex += 1;
+        }
+    }
+    mex
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    struct MexTests {
+        test_arrays: Vec<Vec<i64>>,
+        outputs: Vec<i64>,
+    }
+    impl MexTests {
+        fn new() -> Self {
+            return Self {
+                test_arrays: vec![
+                    vec![-1, 0, 1, 2, 3],
+                    vec![-100, 0, 1, 2, 3, 5],
+                    vec![-1000000, 0, 1, 2, 5],
+                    vec![2, 0, 1, 2, 4],
+                    vec![1, 2, 3, 0, 4],
+                    vec![0, 1, 5, 2, 4, 3],
+                    vec![0, 1, 2, 3, 4, 5, 6],
+                    vec![0, 1, 2, 3, 4, 5, 6, 7],
+                    vec![0, 1, 2, 3, 4, 5, 6, 7, 8],
+                ],
+                outputs: vec![4, 4, 3, 3, 5, 6, 7, 8, 9],
+            };
+        }
+        fn test_function(&self, f: fn(&[i64]) -> i64) {
+            for (nums, output) in self.test_arrays.iter().zip(self.outputs.iter()) {
+                assert_eq!(f(nums), *output);
+            }
+        }
+    }
+    #[test]
+    fn test_mex_using_set() {
+        let tests = MexTests::new();
+        mex_using_set(&[1, 23, 3]);
+        tests.test_function(mex_using_set);
+    }
+    #[test]
+    fn test_mex_using_sort() {
+        let tests = MexTests::new();
+        tests.test_function(mex_using_sort);
+    }
+}
diff --git a/src/general/mod.rs b/src/general/mod.rs
index 4bf38ec6f3f..e719dada390 100644
--- a/src/general/mod.rs
+++ b/src/general/mod.rs
@@ -3,14 +3,16 @@ mod fisher_yates_shuffle;
 mod hanoi;
 mod huffman_encoding;
 mod kmeans;
+mod mex;
 mod nqueens;
 mod two_sum;
-
 pub use self::convex_hull::convex_hull_graham;
 pub use self::fisher_yates_shuffle::fisher_yates_shuffle;
 pub use self::hanoi::hanoi;
 pub use self::huffman_encoding::{HuffmanDictionary, HuffmanEncoding};
 pub use self::kmeans::f32::kmeans as kmeans_f32;
 pub use self::kmeans::f64::kmeans as kmeans_f64;
+pub use self::mex::mex_using_set;
+pub use self::mex::mex_using_sort;
 pub use self::nqueens::nqueens;
 pub use self::two_sum::two_sum;