Efficient way to find Frequency of a character in a String in java : O(n)

Question

In a recent interview I was asked to write the below program. Find out the character whose frequency is minimum in the given String ? So I tried by iterating through the string

Answer 1

Having to iterate through the HashMap is not necessarily bad. That will only be O(h) where h is the HashMap's length--the number of unique characters--which in this case will always be less than or equal to n. For the example "aaabbc", h = 3 for the three unique characters. But, since h is strictly less than the number of possible characters: 255, it is constant. So, your big-oh will be O(n+h) which is actually O(n) since h is constant. I don't know of any algorithm that could get a better big-oh, you could try to have a bunch of java specific optimizations, but that said here is a simple algorithm I wrote that finds the char with the lowest frequency. It returns "c" from the input "aaabbc".

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

public class StackOverflowQuestion {

public static void main(String[] args) {
    // TODO Auto-generated method stub

    System.out.println("" + findLowestFrequency("aaabbc"));

}

public static char findLowestFrequency(String input) {

    Map map = new HashMap();

    for (char c : input.toCharArray())

        if (map.containsKey(c))
            map.put(c, map.get(c) + 1);
        else
            map.put(c, 0);

    char rarest = map.keySet().iterator().next();

    for (char c : map.keySet())

        if (map.get(c) < map.get(rarest))
            rarest = c;

    return rarest;

}

}