Solving problem: First Unique Character

Table of Contents

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• What is the First Unique Character problem
• Examples
• Breaking down
• Visual Representation
• Solution

What is the First Unique Character problem

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It is a coding challenge that requires find the first non-repeating character in the string. If there are no unique characters in the string then you need to return -1.

Examples

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INPUT:
   s = "helloworld"

OUTPUT: 0

INPUT:
   s = "happybirthday"

OUTPUT: 5

Explanation: The unique characters in the string happybirthday are b, i, r, t and d, rest all remaning characters h, a, p and y are non-unique characters. The problem says we have to return the index of the first non-repeating character. In this case, the first non-repeating character is b and its index is 5.

h => 0
a => 1
p => 2
p => 3
y => 4
b => 5

Breaking down

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This approach includes breaking down complex problem in to smaller and smaller sub-problems. For example — You want to watch movie in theater.

• Book a ticket
• You will go the theater
• Need to pass the security check
• Need to show your booking ticket at the counter
• so on...

 

The benefit of using this approach is that you get a clear and better understanding of the question. Its obvious, it you know the question better you can come up with a better and efficient solution for the problem.

Visual Representation

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Visual Representation is another way of finding solution to the problem. Let's take a look how to find First Unique Character in the string.

• Looping through all the characters in the string.
• Checking if the character already exists in the hash.
  • then — removing it from the hash and adding it to duplicate hash.
  • else   — add current index to hash with the key as character

Solution

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var firstUniqChar = function (s) {
   const del = {};
   const hash = {};

   for (let i = 0; i < s.length; i++) {
      let k = s[i];

      if (hash[k] != undefined) {
         del[k] = i;
         delete hash[k];
      }

      if (del[k] != undefined) {
         continue;
      }

      hash[k] = i;
   }

   return hash[Object.keys(hash)[0]] ?? -1;
};

 

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