docs: add algorithm description for three sum problems

Author: bujosa

src/medium/three_sum.rs

@@ -1,3 +1,4 @@
+#![allow(dead_code)]
 pub fn three_sum(nums: Vec<i32>) -> Vec<Vec<i32>> {
     let mut nums = nums;
     nums.sort();
@@ -30,6 +31,12 @@ pub fn three_sum(nums: Vec<i32>) -> Vec<Vec<i32>> {
     res
 }
 
+/*
+    Algorithm - Two Pointers
+        - Time Complexity: O(n^2)
+        - Space Complexity: O(1)
+*/
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -43,4 +50,4 @@ mod tests {
     }
 }
 
-// Reference: https://leetcode.com/problems/3sum/
+// Reference: https://leetcode.com/problems/3sum/

src/medium/three_sum_closest.rs

@@ -1,3 +1,4 @@
+#![allow(dead_code)]
 pub fn three_sum_closest(nums: Vec<i32>, target: i32) -> i32 {
     let mut nums = nums;
     nums.sort();
@@ -22,6 +23,12 @@ pub fn three_sum_closest(nums: Vec<i32>, target: i32) -> i32 {
     closest
 }
 
+/*
+    Algorithm - Two Pointers
+        - Time Complexity: O(n^2)
+        - Space Complexity: O(1)
+*/
+
 #[test]
 fn test_three_sum_closest() {
     assert_eq!(three_sum_closest(vec![-1, 2, 1, -4], 1), 2);