Add Merge Alternately

Merge_Strings_Alternately/merge_strings_alternately.py

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+"""
+Problem: https://leetcode.com/problems/merge-strings-alternately/
+
+You are given two strings, word1 and word2. Merge the strings by adding letters in alternating order,
+starting with word1. If one string is longer, append the remaining letters of the longer string
+to the merged result.
+
+Example 1:
+Input: word1 = "abc", word2 = "pqr"
+Output: "apbqcr"
+
+Example 2:
+Input: word1 = "ab", word2 = "pqrs"
+Output: "apbqrs"
+
+Example 3:
+Input: word1 = "abcd", word2 = "pq"
+Output: "apbqcd"
+
+Constraints:
+- 1 <= len(word1), len(word2) <= 100
+- word1 and word2 consist of lowercase English letters only.
+"""
+
+# -------------------- FIRST ATTEMPT --------------------
+# Approach: Loop up to the minimum length of the two strings,
+# appending characters alternately. Then, append any remaining substring.
+
+def mergeAlternately(word1: str, word2: str) -> str:
+    # Find the smallest length between the two words
+    minLen = min(len(word1), len(word2))
+
+    # Initialize an empty string to store the result
+    new_word = ""
+
+    # Loop through indices up to the smaller length
+    for i in range(minLen):
+        new_word += word1[i]  # Add character from word1
+        new_word += word2[i]  # Add character from word2
+
+    # If word1 is longer, append the remaining characters
+    if len(word1) > minLen:
+        new_word += word1[minLen:]
+
+    # If word2 is longer, append its remaining characters
+    elif len(word2) > minLen:
+        new_word += word2[minLen:]
+
+    # Return the merged result
+    return new_word
+
+
+"""
+Thought Process:
+
+This first solution is intuitive and readable. It loops up to the shorter string’s length, 
+then concatenates the leftovers from the longer one.
+
+However, each time you use the '+' operator on strings inside a loop, Python creates a *new string* 
+(because strings are immutable). This results in higher time complexity.
+
+   Analysis of Algorithm:
+    - Time Complexity: O(n²) due to repeated string concatenations
+    - Space Complexity: O(n + m)
+    - Readability: Clear and simple
+
+   Problem:
+    - Not optimal for longer strings.
+    - Each concatenation creates a temporary copy → inefficient.
+"""
+
+# -------------------- OPTIMIZED VERSION --------------------
+# Approach: Use a list to accumulate characters, then join at the end.
+# This avoids multiple string copies and runs in linear time.
+
+def mergeAlternatelyOptimized(word1: str, word2: str) -> str:
+    """Optimized: Merge two strings alternately using list accumulation."""
+
+    maxLen = max(len(word1), len(word2))  # Loop up to the longer string length
+    new_arr = []  # Use a list to efficiently collect characters
+
+    for i in range(maxLen):
+        if i < len(word1):  # Check boundary for word1
+            new_arr.append(word1[i])
+        if i < len(word2):  # Check boundary for word2
+            new_arr.append(word2[i])
+
+    # Join list into a single string at the end
+    return ''.join(new_arr)
+
+
+"""
+Optimized Approach Discussion:
+
+ Idea:
+    Instead of concatenating strings in every loop iteration, 
+    store all characters in a list and join once at the end.
+
+ Why This Works:
+    - Lists are mutable → appending is O(1) amortized.
+    - `''.join(list)` efficiently combines all elements in one pass.
+
+ Analysis of Algorithm:
+    - Time Complexity: O(n + m)
+    - Space Complexity: O(n + m)
+    - Readability:  Still simple and elegant
+    - Performance:  Much faster for longer inputs
+
+"""
+# Driver code
+def main():
+    word1 = "abcd"
+    word2 = "pq"
+    print(mergeAlternatelyOptimized(word1, word2))  # Output: "apbqcd"
+
+if __name__ == "__main__":
+    main()