Approach 1: Linear time solution
The best possible solution here could be of a linear time because to ensure that the character is unique you have to check the whole string anyway.
The idea is to go through the string and
save in a hash map the number of times
each character appears in the string.
That would take time,
N is a number of characters in the string.
And then we go through the string the second time, this time
we use the hash map as a reference to check if a character
is unique or not.
If the character is unique, one could just return its index. The complexity of the second iteration is as well.
- Time complexity : since we go
through the string of length
- Space complexity : because English alphabet contains 26 letters.