LeetCode 387: Finding the First Unique Character - Frequency Mapping in Java

Difficulty: Easy

Topics: Hash Table, String, Queue, Counting

Platform: Leetcode

Problem Statement

Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1.

Problem Statement Simplified
Get the index of the first non-repeating character, else return -1.

Mistakes and Learning
Do not blindly loop thorugh.
Do not just delete the duplicates, remove the duplicated character also.

Example 1

`Input: s = "leetcode"

Output: 0

Explanation:

The character 'l' at index 0 is the first character that does not occur at any other index.`

Example 2

`Input: s = "loveleetcode"

Output: 2`

Key Insight

Remove all the duplicating characters
Then get the first character
Get the first character index.
If no unique then -1.

Algorithm

Convert the string to a char array.
Intitialize a HashMap to store the character and the repeating counts.
Intitialize a for loop
Put the character in the HashMap with getOrDefault to check if the character already preset or not, if yes then increment by 1.
End for loop
Intitialize a for loop
Check if the first value is 1 if yes then return the for loop index else continue the for loop
End for loop
return -1

Algorithm in simple words

First, convert the string to a char array. Then initialize a HashMap so that we can store each character with its repeating counts.
Then initialize a for loop to add each character from character array to HashMap with getOrDefault ( if the value of preseent the increment b 1 else it will be 0 ), end of for loop. Then initialize another for loop to check the first character with value 1 from HashMap. if found then return the loop index else it will end the loop and return -1.

Java code

class Solution {
    public int firstUniqChar(String s) {
        char[] character = s.toCharArray();
        Map <Character, Integer> map= new HashMap<>();
        for(int i=0;i<character.length;i++){
            map.put(character[i],map.getOrDefault(character[i],0)+1);
        }
       for(int i=0;i<character.length;i++){
        if(map.get(character[i])==1){
            return i;
        }
       }
        return -1;
    }
}