Add longest_uniform_substring_after_replacements

Author: marttp

rust/Sliding Windows/longest_uniform_substring_after_replacements.rs

@@ -0,0 +1,35 @@
+fn longest_uniform_substring_after_replacements(s: String, k: i32) -> i32 {
+    use std::collections::HashMap;
+
+    let mut freqs = HashMap::new();
+    let mut highest_freq = 0;
+    let mut max_len = 0;
+    let mut left = 0;
+    let mut right = 0;
+    let s = s.as_bytes();
+
+    while right < s.len() {
+        // Update the frequency of the character at the right pointer
+        // and the highest frequency for the current window.
+        *freqs.entry(s[right]).or_insert(0) += 1;
+        highest_freq = highest_freq.max(*freqs.get(&s[right]).unwrap());
+        // Calculate replacements needed for the current window.
+        let num_chars_to_replace = ((right - left + 1) as i32) - highest_freq;
+        // Slide the window if the number of replacements needed exceeds
+        // 'k'. The right pointer always gets advanced, so we just need
+        // to advance 'left'.
+        if num_chars_to_replace > k {
+            // Remove the character at the left pointer from the hash map
+            // before advancing the left pointer.
+            *freqs.get_mut(&s[left]).unwrap() -= 1;
+            left += 1;
+        }
+        // Since the length of the current window increases or stays the
+        // same, assign  the length of the current window to 'max_len'.
+        max_len = max_len.max(right - left + 1);
+        // Expand the window.
+        right += 1;
+    }
+
+    max_len as i32
+}