diff --git a/src/main.rs b/src/main.rs
index 3c45ce2..84fdd64 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -97,6 +97,6 @@ fn main() {
     // c2a1();
     // c2a2();
     // c2a3();
-    c2a4();
-    // c3a1();
+    // c2a4();
+    c3a1();
 }
diff --git a/src/two_sum.rs b/src/two_sum.rs
index dfc161c..9f36c64 100644
--- a/src/two_sum.rs
+++ b/src/two_sum.rs
@@ -1,5 +1,45 @@
 use std::collections::HashSet;
-// use std::iter::FromIterator;
+
+fn binary_search<T: std::cmp::PartialOrd>(min_i: usize, max_i: usize, xs: &Vec<T>, needle: T) -> usize {
+	if min_i >= max_i {
+		return max_i;
+	}
+	let half_i = min_i + (max_i - min_i) / 2;
+
+	if xs[half_i] == needle {
+		return half_i;
+	}
+
+	if needle < xs[half_i] {
+		binary_search(min_i, half_i - 1, &xs, needle)
+	} else {
+		binary_search(half_i + 1, max_i, xs, needle)
+	}
+}
+
+pub fn find_two_sums(xs: &mut Vec<i64>) -> usize {
+	let mut found: HashSet<i64> = HashSet::new();
+	const T_MIN: i64 = -10000;
+	const T_MAX: i64 = 10000;
+	xs.sort();
+
+	for i in 0..xs.len() {
+		let a: i64 = xs[i];
+		let b_min = T_MIN - a;
+		let b_max = T_MAX - a;
+		let i_min = binary_search(i + 1, xs.len() - 1, &xs, b_min);
+		let i_max = binary_search(i + 1, xs.len() - 1, &xs, b_max);
+
+		for i in i_min..(i_max + 1) {
+			let b = xs[i];
+			let t = a + b;
+			if T_MIN <= t && t <= T_MAX {
+				found.insert(t);
+			}
+		}
+	}
+	found.len()
+}
 
 // fn two_sum(t: i64, xs: &Vec<i64>, ys: &HashSet<i64>) -> bool {
 // 	for &x in xs {
@@ -11,61 +51,7 @@ use std::collections::HashSet;
 // 	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;
-		}
-	}
-
-	0
-}
-
-// 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);
-
-// }
-
-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);
-	}
-	found.len()
-}
-
-// pub fn find_two_sums(xs: Vec<i64>) -> usize {
-
+// pub fn find_two_sums_naiv(xs: Vec<i64>) -> usize {
 // 	let mut xs_min: Vec<i64> = xs.to_vec();
 // 	xs_min.retain(|&i| i < 0);
 
@@ -84,5 +70,3 @@ pub fn find_two_sums(xs: &mut Vec<i64>) -> usize {
 // 	}
 // 	r
 // }
-
-