3 ways to Find First Non Repeated Character in a String - Java Programming Problem

Write a Java program to find first non repeated character in a String is a common question on coding tests. Since String is a popular topic in various programming interviews, It's better to prepare well with some well-known questions like reversing String using recursion, or checking if a String is palindrome or not. This question is also in the same league. Before jumping into solution, let's first understand this question. You need to write a function, which will accept a String and return first non-repeated character, for example in world "hello" , except 'l' all are non-repeated, but 'h' is the first non-repeated character. Similarly in word "swiss" 'w' is the first non-repeated character. One way to solve this problem is creating a table to store count of each character, and then picking the first entry which is not repeated. Key thing to remember is order, your code must return first non-repeated letter. By the way, In this article, we will see 3 examples to find first non repeated character from a String. Our first solution uses LinkedHashMap to store character count, since LinkedHashMap maintains insertion order and we are inserting character in the order they appear in String, once we scanned String, we just need to iterate through LinkedHashMap and choose the entry with value 1. Yes, this solution require one LinkedHashMap and two for loops. Our second solution is a trade-off between time and space, to find first non repeated character in one pass. This time, we have used one Set and one List to keep repeating and non-repeating character separately. Once we finish scanning through String, which is O(n), we can get the magic character by accessing List which is O(1) operator. Since List is an ordered collection get(0) returns first element. Our third solution is also similar, but this time we have used HashMap instead of LinkedHashMap and we loop through String again to find first non-repeated character. In next section, we will the code example and unit test for this programming question. You can also see my list of String interview Questions for more of such problems and questions from Java programming language.

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