diff --git a/.gitignore b/.gitignore
index 6f6f9cd..846f9fc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,3 +16,4 @@ algos.sublime-workspace
 
 # Data for the exercises. Some files are big so don't check them in.
 data/c2a4.txt
+data/c2a4.back.txt
diff --git a/Cargo.toml b/Cargo.toml
index 0936d0f..830a0a0 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,4 +7,7 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-rand = "0.7.3"
\ No newline at end of file
+rand = "0.7.3"
+
+[profile.release]
+opt-level = 3
\ No newline at end of file
diff --git a/misc/prims_mst_1.png b/misc/prims_mst_1.png
new file mode 100644
index 0000000..728f3a0
Binary files /dev/null and b/misc/prims_mst_1.png differ
diff --git a/misc/prims_mst_2.png b/misc/prims_mst_2.png
new file mode 100644
index 0000000..802be69
Binary files /dev/null and b/misc/prims_mst_2.png differ
diff --git a/misc/prims_mst_3.png b/misc/prims_mst_3.png
new file mode 100644
index 0000000..9f7f9b6
Binary files /dev/null and b/misc/prims_mst_3.png differ
diff --git a/src/main.rs b/src/main.rs
index 37b17fa..3c45ce2 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -16,6 +16,7 @@ use crate::quick_sort::quick_sort;
 use crate::ssc::ssc;
 use crate::two_sum::find_two_sums;
 
+
 #[allow(dead_code)]
 fn c1a2() {
     let vec = util::read_to_vector("data/c1a2.txt").unwrap();
@@ -78,9 +79,10 @@ fn c2a3() {
 
 #[allow(dead_code)]
 fn c2a4() {
-    let v = util::read_to_vector("data/c2a4.txt").unwrap();
-    let r = find_two_sums(v);
+    let mut v = util::read_to_vector("data/c2a4.txt").unwrap();
+    let r = find_two_sums(&mut v);
     println!("c2a4={:?}", r);
+	// c2a4=427
 }
 
 #[allow(dead_code)]
@@ -95,6 +97,6 @@ fn main() {
     // c2a1();
     // c2a2();
     // c2a3();
-    // c2a4();
-    c3a1();
+    c2a4();
+    // c3a1();
 }
diff --git a/src/two_sum.rs b/src/two_sum.rs
index 495d3a4..dfc161c 100644
--- a/src/two_sum.rs
+++ b/src/two_sum.rs
@@ -1,32 +1,88 @@
 use std::collections::HashSet;
-use std::iter::FromIterator;
+// use std::iter::FromIterator;
 
-fn two_sum(t: i64, xs: &Vec<i64>, ys: &HashSet<i64>) -> bool {
-	for &x in xs {
-		let y = t - x;
-		if x != y && ys.contains(&y) {
-			return true;
+// fn two_sum(t: i64, xs: &Vec<i64>, ys: &HashSet<i64>) -> bool {
+// 	for &x in xs {
+// 		let y = t - x;
+// 		if x != y && ys.contains(&y) {
+// 			return true;
+// 		}
+// 	}
+// 	false
+// }
+
+fn binary_search(min_i: usize, xs: &Vec<i64>, needle: i64) -> usize {
+	let mut width = xs.len() - min_i;
+	let mut i = min_i + width / 2;
+
+	if xs[i] == needle {
+		return i;
+	}
+
+	while width > 1 {
+
+		width = width / 2;
+
+		if xs[i] < needle {
+			i = i + width;
+		} else if xs[i] > needle {
+			i = i - width;
+
+		} else {
+			return i;
 		}
 	}
-	false
+
+	0
 }
 
-pub fn find_two_sums(xs: Vec<i64>) -> usize {
+// fn add_two_sums_for_limits(i: usize, xs: &Vec<i64>) -> () {
+// 	let t_min = -10000;
+// 	let t_max = 10000;
+// 	let a: i64 = xs[i];
+// 	let b_min = t_min - a;
+// 	let b_max = t_max - a;
+// 	println!("{:?} {:?}", b_min, b_max);
 
-	let mut xs_min: Vec<i64> = xs.to_vec();
-	xs_min.retain(|&i| i < 0);
+// }
 
-	let mut xs_max: Vec<i64> = xs.to_vec();
-	xs_max.retain(|&i| i >= 0);
-	let xs_max_set: HashSet<i64> = HashSet::from_iter(xs_max);
-
-	let mut r: usize = 0;
-	for t in -10000..10001 {
-		if two_sum(t, &xs_min, &xs_max_set) {
-			r += 1
-		}
+pub fn find_two_sums(xs: &mut Vec<i64>) -> usize {
+	let found: HashSet<i64> = HashSet::new();
+	const T_MIN: i64 = -3;
+	const T_MAX: i64 = 3;
+	xs.sort();
+	for i in 0..1 {
+		// add_two_sums_for_limits(i, &xs);
+		let a: i64 = xs[i];
+		let b_min = T_MIN - a;
+		let b_max = T_MAX - a;
+		println!("a={} b_min={} b_max={}", a, b_min, b_max);
+		let i_min = binary_search(i, &xs, b_min);
+		let i_max = binary_search(i, &xs, b_max);
+		println!("{:?} {:?}", i_min, i_max);
 	}
-	r
+	found.len()
 }
 
+// pub fn find_two_sums(xs: Vec<i64>) -> usize {
+
+// 	let mut xs_min: Vec<i64> = xs.to_vec();
+// 	xs_min.retain(|&i| i < 0);
+
+// 	let mut xs_max: Vec<i64> = xs.to_vec();
+// 	xs_max.retain(|&i| i >= 0);
+// 	let xs_max_set: HashSet<i64> = HashSet::from_iter(xs_max);
+
+// 	let mut r: usize = 0;
+// 	for t in -10000..10001 {
+// 		if t % 10 == 0 {
+// 			println!("{:?}", t);
+// 		} 
+// 		if two_sum(t, &xs_min, &xs_max_set) {
+// 			r += 1
+// 		}
+// 	}
+// 	r
+// }
+