Algorithm and program to find out duplicate character in a String

🧮 Find Duplicate Characters in a String (Java)

🧩 What problem are we solving?

The goal is to analyze a given string and determine how many times each character appears — effectively identifying duplicate or repeating characters.

For example, given:

"My name is Vinod"

the output should display how often each character occurs:

Character :   Count : 3
Character : a Count : 1
Character : s Count : 1
Character : d Count : 1
Character : e Count : 1
Character : V Count : 1
Character : y Count : 1
Character : i Count : 2
Character : M Count : 1
Character : m Count : 1
Character : n Count : 2
Character : o Count : 1

This problem is useful for understanding:

• How to work with maps (HashMap)

• How to iterate characters in a string

• How to count frequencies of elements efficiently

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⚙️ Algorithm (Step-by-Step)

1. Create a HashMap<Character, Integer>
   → To store each character as a key and its count as the value.

2. Convert the string into a character array
   → Use toCharArray() to easily loop through characters.

3. Iterate through the character array
   → For each character:

   • If it’s already in the map → increment the count.

   • If not → add it with count = 1.

4. Display the results
   → Print each key-value pair to show how many times each character occurs.

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🧠 Example Walkthrough

Let’s trace through the string:
"My name is Vinod"

Step	Character	Already in Map?	Action	Updated Count
1	M	❌	Add M=1	1
2	y	❌	Add y=1	1
3	(space)	❌	Add space=1	1
4	n	❌	Add n=1	1
5	a	❌	Add a=1	1
6	m	❌	Add m=1	1
7	e	❌	Add e=1	1
8	(space)	✅	Increment	2
9	i	❌	Add i=1	1
10	s	❌	Add s=1	1
11	(space)	✅	Increment	3
12	V	❌	Add V=1	1
13	i	✅	Increment	2
14	n	✅	Increment	2
15	o	❌	Add o=1	1
16	d	❌	Add d=1	1

✅ Final Map contents printed in iteration order:

(space)=3, a=1, s=1, d=1, e=1, V=1, y=1, i=2, M=1, m=1, n=2, o=1

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📘 Java Implementation

package com.vinod.test;

import java.util.HashMap;
import java.util.Map;

/**
 * FindDuplicateCharactorInString
 *
 * Problem:
 *   Count and identify duplicate characters in a given string.
 *
 * Algorithm:
 *   1. Create a HashMap to store each character and its frequency.
 *   2. Convert the input string into a char[] array.
 *   3. Iterate through the array:
 *        - If the character exists in the map, increment its count.
 *        - Otherwise, add it to the map with count = 1.
 *   4. Print all characters and their counts.
 *
 * Example:
 *   Input:  "My name is Vinod"
 *   Output:
 *     Character :   Count : 3
 *     Character : a Count : 1
 *     Character : s Count : 1
 *     Character : d Count : 1
 *     Character : e Count : 1
 *     Character : V Count : 1
 *     Character : y Count : 1
 *     Character : i Count : 2
 *     Character : M Count : 1
 *     Character : m Count : 1
 *     Character : n Count : 2
 *     Character : o Count : 1
 *
 * Time Complexity:
 *   O(n) – Each character is processed once.
 *
 * Space Complexity:
 *   O(k) – Where k is the number of unique characters.
 *
 * Author: Vinod Kariyathungal Kumaran
 */
public class FindDuplicateCharactorInString {

    public static void main(String[] args) {
        Map duplicateMap = new HashMap<>();

        String str = "My name is Vinod";
        char[] chrs = str.toCharArray();

        for (Character ch : chrs) {
            if (duplicateMap.containsKey(ch)) {
                duplicateMap.put(ch, duplicateMap.get(ch) + 1);
            } else {
                duplicateMap.put(ch, 1);
            }
        }

        duplicateMap.forEach((k, v) ->
            System.out.println("Character : " + k + " Count : " + v)
        );
    }
} 

Character :   Count : 3
Character : a Count : 1
Character : s Count : 1
Character : d Count : 1
Character : e Count : 1
Character : V Count : 1
Character : y Count : 1
Character : i Count : 2
Character : M Count : 1
Character : m Count : 1
Character : n Count : 2
Character : o Count : 1