feat: Add Minimum Window Substring algorithm in R (#165)

Author: sgindeed

string_manipulation/minimum.window.substring.R

@@ -0,0 +1,66 @@
+# Minimum Window Substring in R
+# Author: sgindeed
+# Description: Finds the smallest substring of s that contains all characters of t
+
+# Ask for inputs
+s <- readline(prompt = "Enter main string: ")
+t <- readline(prompt = "Enter target characters: ")
+
+# Convert to lowercase for case-insensitivity
+s <- tolower(s)
+t <- tolower(t)
+
+# Edge case
+if (nchar(s) == 0 || nchar(t) == 0) {
+  cat("Empty input. Exiting.\n")
+  quit(save = "no")
+}
+
+# Convert to char arrays
+s_chars <- strsplit(s, "")[[1]]
+t_chars <- strsplit(t, "")[[1]]
+
+# Frequency of characters in t
+t_count <- table(t_chars)
+window_count <- list()
+
+required <- length(t_count)
+formed <- 0
+
+left <- 1
+right <- 0
+min_len <- Inf
+min_window <- ""
+
+# Sliding window
+while (right < length(s_chars)) {
+  right <- right + 1
+  char <- s_chars[right]
+  window_count[[char]] <- (window_count[[char]] %||% 0) + 1
+  
+  if (!is.na(t_count[char]) && window_count[[char]] == t_count[char]) {
+    formed <- formed + 1
+  }
+  
+  # Try to contract the window
+  while (left <= right && formed == required) {
+    if ((right - left + 1) < min_len) {
+      min_len <- right - left + 1
+      min_window <- paste0(s_chars[left:right], collapse = "")
+    }
+    
+    left_char <- s_chars[left]
+    window_count[[left_char]] <- window_count[[left_char]] - 1
+    if (!is.na(t_count[left_char]) && window_count[[left_char]] < t_count[left_char]) {
+      formed <- formed - 1
+    }
+    left <- left + 1
+  }
+}
+
+if (is.infinite(min_len)) {
+  cat("No valid window found.\n")
+} else {
+  cat("Minimum window substring:", min_window, "\n")
+  cat("Length:", min_len, "\n")
+}