Added find duplicate characters problem in python string section.

algorithms/Python/README.md

@@ -67,6 +67,7 @@
 - [Rabin Karp algorithm](strings/rabin-karp-algorithm.py)
 - [Find all permutations](strings/find_all_permutations.py)
 - [Roman to Int](strings/roman-to-int.py)
+- [Find duplicate characters](strings\find_duplicate_characters.py)
 
 ## Dynamic Programming
 - [Print Fibonacci Series Up To N-th Term](dynamic_programming/fibonacci_series.py)

algorithms/Python/strings/find_duplicate_characters.py

@@ -0,0 +1,51 @@
+"""
+Problem:
+Given a string, find all the characters that occur more than once, ignoring the case and whitespaces.
+Given a string inp_str, return a list of strings containing characters that occur more than once, ignoring the case.
+You can return characters in any order, if no duplicates are found return an empty list [].
+
+
+Solution:
+In this solution maintain a dictionary for the count of occurence of characters, ignoring the case. Dictionary 
+will have the keys as the characters and their value as the count of occurrence in string.
+
+We will iterate over the string the and will check whether chracter exists in dictionary or not.
+If character exists, we will increment the count of charcter (value) by 1.
+If character does not exists, we will add a new key in the dictionary with key as character and value as 1.
+
+We will return the list of characters from the dictinary keys for whom value is greater than 1.
+
+Complexity Analysis:
+Time complexity: O(n). We traverse the string containing n elements only once. 
+Space complexity: O(n). The extra space required depends on the number of items stored in the dictionary,
+which stores at most n elements
+"""
+
+
+def duplicate_characters(inp_str):
+    dictionary = {}
+    for char in inp_str:
+        if char in dictionary.keys():
+            dictionary[char] += 1
+        else:
+            dictionary[char] = 1
+
+    duplicates = []
+
+    for char, count in dictionary.items():
+        if count > 1:
+            duplicates.append(char)
+
+    return duplicates
+
+
+def main():
+    try:
+        inp_str = input("Enter input string: ").strip().replace(" ","").lower()
+    except ValueError:
+        inp_str = 'testingstring'
+    print(duplicate_characters(inp_str))
+
+
+if __name__ == "__main__":
+    main()