From 04cc50e082bca1e476fa1a749c5f932f1c597462 Mon Sep 17 00:00:00 2001
From: Rajas Paranjpe <52586855+ChocolateLoverRaj@users.noreply.github.com>
Date: Mon, 27 Apr 2026 23:22:04 -0700
Subject: [PATCH 1/2] Improve Zig Zag conversion

Single allocation, no reallocations

This takes advantage of each character always being 1 byte, which is
guaranteed by the LeetCode constraints.
---
 src/leetcode/zigzag_conversion.rs | 78 ++++++++++++++++++++-----------
 1 file changed, 52 insertions(+), 26 deletions(-)

diff --git a/src/leetcode/zigzag_conversion.rs b/src/leetcode/zigzag_conversion.rs
index 847e2ff..8e57fac 100644
--- a/src/leetcode/zigzag_conversion.rs
+++ b/src/leetcode/zigzag_conversion.rs
@@ -1,32 +1,56 @@
-fn add(u: usize, i: i32) -> Option<usize> {
-    if i.is_negative() {
-        u.checked_sub(i.wrapping_abs() as u32 as usize)
-    } else {
-        u.checked_add(i as usize)
+pub fn convert(s: String, num_rows: i32) -> String {
+    let num_rows = num_rows as usize;
+    // We know the final len of the string so there will be no reallocation or wasted memory
+    let mut o = String::with_capacity(s.len());
+    let step = match num_rows {
+        1 => 1,
+        num_rows => num_rows * 2 - 2,
+    };
+    // Append the top row
+    {
+        let mut i = 0;
+        loop {
+            if let Some(c) = s.as_bytes().get(i * step).copied() {
+                o.push(char::from(c));
+            } else {
+                break;
+            }
+            i += 1;
+        }
     }
-}
-
-pub fn convert(s: String, n: i32) -> String {
-    if n == 1 || n as usize >= s.len() {
-        return s;
+    // Append middle rows
+    for row in 1..num_rows - 1 {
+        let start_0 = row;
+        let start_1 = num_rows * 2 - 2 - row;
+        let mut i = 0;
+        loop {
+            if let Some(c) = s.as_bytes().get(start_0 + i * step).copied() {
+                o.push(char::from(c));
+            } else {
+                break;
+            }
+            if let Some(c) = s.as_bytes().get(start_1 + i * step).copied() {
+                o.push(char::from(c));
+            } else {
+                break;
+            }
+            i += 1;
+        }
     }
-
-    let mut rows = vec![String::new(); n as usize];
-
-    let mut row_index = 0;
-    let mut step = 1;
-    for c in s.chars() {
-        rows[row_index].push(c);
-        if row_index == 0 {
-            step = 1;
-        } else if row_index == n as usize - 1 {
-            step = -1;
+    // Append the last row
+    if num_rows > 1 {
+        let start = num_rows - 1;
+        let mut i = 0;
+        loop {
+            if let Some(c) = s.as_bytes().get(start + i * step).copied() {
+                o.push(char::from(c));
+            } else {
+                break;
+            }
+            i += 1;
         }
-
-        row_index = add(row_index, step).unwrap();
     }
-
-    rows.join("")
+    o
 }
 
 #[cfg(test)]
@@ -43,6 +67,8 @@ mod tests {
         assert_eq!(
             "PINALSIGYAHRPI".to_string(),
             convert("PAYPALISHIRING".to_string(), 4)
-        )
+        );
+
+        assert_eq!("A".to_string(), convert("A".to_string(), 1));
     }
 }

From 50610e0f2926273b7ba22134863ed246da737aa2 Mon Sep 17 00:00:00 2001
From: jusexton <justinsexton.dev@gmail.com>
Date: Wed, 6 May 2026 22:37:43 -0500
Subject: [PATCH 2/2] fix linting

---
 src/leetcode/zigzag_conversion.rs | 66 ++++++++++++-------------------
 1 file changed, 26 insertions(+), 40 deletions(-)

diff --git a/src/leetcode/zigzag_conversion.rs b/src/leetcode/zigzag_conversion.rs
index 8e57fac..ac4bf2f 100644
--- a/src/leetcode/zigzag_conversion.rs
+++ b/src/leetcode/zigzag_conversion.rs
@@ -1,55 +1,41 @@
 pub fn convert(s: String, num_rows: i32) -> String {
-    let num_rows = num_rows as usize;
-    // We know the final len of the string so there will be no reallocation or wasted memory
-    let mut o = String::with_capacity(s.len());
-    let step = match num_rows {
-        1 => 1,
-        num_rows => num_rows * 2 - 2,
-    };
-    // Append the top row
-    {
+    fn append(o: &mut String, bytes: &[u8], start: usize, step: usize) {
         let mut i = 0;
-        loop {
-            if let Some(c) = s.as_bytes().get(i * step).copied() {
-                o.push(char::from(c));
-            } else {
-                break;
-            }
+        while let Some(&c) = bytes.get(start + i * step) {
+            o.push(c as char);
             i += 1;
         }
     }
-    // Append middle rows
-    for row in 1..num_rows - 1 {
-        let start_0 = row;
-        let start_1 = num_rows * 2 - 2 - row;
+
+    let num_rows = num_rows as usize;
+    let bytes = s.as_bytes();
+    let step = match num_rows {
+        1 => 1,
+        n => n * 2 - 2,
+    };
+    let mut o = String::with_capacity(s.len());
+
+    // Top row
+    append(&mut o, bytes, 0, step);
+
+    // Middle rows
+    for row in 1..num_rows.saturating_sub(1) {
         let mut i = 0;
-        loop {
-            if let Some(c) = s.as_bytes().get(start_0 + i * step).copied() {
-                o.push(char::from(c));
-            } else {
-                break;
-            }
-            if let Some(c) = s.as_bytes().get(start_1 + i * step).copied() {
-                o.push(char::from(c));
-            } else {
-                break;
+        while let Some(&a) = bytes.get(row + i * step) {
+            o.push(a as char);
+            match bytes.get((step - row) + i * step).copied() {
+                Some(b) => o.push(b as char),
+                None => break,
             }
             i += 1;
         }
     }
-    // Append the last row
+
+    // Bottom row
     if num_rows > 1 {
-        let start = num_rows - 1;
-        let mut i = 0;
-        loop {
-            if let Some(c) = s.as_bytes().get(start + i * step).copied() {
-                o.push(char::from(c));
-            } else {
-                break;
-            }
-            i += 1;
-        }
+        append(&mut o, bytes, num_rows - 1, step);
     }
+
     o
 }