chore(Python): add string algorithms (#565)

Co-authored-by: Ming Tsai <37890026+ming-tsai@users.noreply.github.com>

algorithms/Go/README.md

@@ -7,6 +7,7 @@
 - [Contains Duplicate](arrays/contains_duplicate.go)
 - [Single Number](arrays/single-number.go)
 
+
 ## Scheduling
 - [Interval Scheduling](scheduling/interval-scheduling.go)
 
@@ -25,3 +26,4 @@
 
 ## String
 - [Palindrome Permutation](strings/palindrome-permutation.go)
+

algorithms/Python/README.md

@@ -48,6 +48,7 @@
 - [First Non Repeating Character](strings/first_non_repeating_character.py)
 - [Longest Common Subsequence](strings/longest_common_subsequence.py)
 - [Unique Character](strings/unique_character.py)
+- [Add String](strings/add_string.py)
 
 ## Dynamic Programming
 - [Print Fibonacci Series Up To N-th Term](dynamic_programming/fibonacci_series.py)
@@ -56,9 +57,6 @@
 - [Catalan Sequence](dynamic_programming/catalan_sequence.py)
 - [0/1 Knapsack Problem](dynamic_programming/knapsack.py)
 
-## Trees
-- [Binary Tree](trees/binary_tree.py)
-
 ## Graphs
 - [Simple Graph](graphs/graph.py)
 

algorithms/Python/strings/add_string.py

@@ -0,0 +1,42 @@
+"""
+Given two non-negative integers, num1 and num2 represented as string, return the sum of num1 and num2 as a string.
+
+You must solve the problem without using any built-in library for handling large integers (such as BigInteger). 
+
+You must also not convert the inputs to integers directly.
+
+Time: O(max(len(num1), len(num2)))
+Space: O(max(len(num1), len(num2)) + 1)
+
+Key points: Traverse reversely of both strings
+"""
+
+def add_string(num1, num2):
+    carry = 0
+    ptr1, ptr2 = len(num1) - 1, len(num2) - 1
+    result = ""
+    
+    while ptr1 >= 0 or ptr2 >= 0 or carry:
+        if ptr1 >= 0:
+            carry += (ord(num1[ptr1]) - ord("0"))
+            ptr1 -= 1
+        if ptr2 >= 0:
+            carry += (ord(num2[ptr2]) - ord('0'))
+            ptr2 -= 1
+        
+        result = str(carry % 10) + result
+        carry //= 10
+    return result
+
+
+if __name__ == "__main__": 
+    num1 = "11"
+    num2 = "123"
+    result = add_string(num1, num2) #expected result  "134"
+
+    print("First test: ",result)
+
+    num1 = "456"
+    num2 = "77"
+    result = add_string(num1, num2) #expected result  "533"    
+    print("Second test: ",result)